MEDICAL
BIOLOGY
MEDICAL
BIOLOGY
Chapter 1. Introduction into the course of medical biology. Levels of organization and fundamental characteristics of living matter. Chemical composition of the cell
1.1. Introduction into the course of medical biology Biology is the science of life and living organisms. The word biology comes from the Greek word: bios – “life” and the suffix-logia-“study of”. It is the study of life and living organisms, including their structure, function, growth, evolution, distribution, and taxonomy. Biology is a vast rapidly developing subject, composed of many branches and disciplines, such as molecular biology, cytology, genetics, parasitology, ecology and many other subjects. In general, biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that moves the synthesis and creation of new species. Medical biology (Biomedicine) is a field of biology that has practical applications in medicine, health care and laboratory diagnostics. It concerns a wide range of scientific and technological approaches: from an in vitro diagnostics to the in vitro fertilization, from the molecular mechanisms of a cystic fibrosis to the population dynamics of the HIV, from the understanding molecular interactions to the study of the carcinogenesis, from a single-nucleotide polymorphism (SNP) to the gene therapy. It includes many biomedical disciplines and areas that typically contain the “bio-” prefix such as: `` Molecular biology, biochemistry, biophysics, biotechnology, cell biology, embryo logy. `` Nanobiotechnology, biological engineering, laboratory medical biology. `` Cytogenetics, Genetics, Gene therapy. `` Bioinformatics, Biostatistics, Systems biology. `` Microbiology, Virology, Parasitology. `` Ecology. `` Physiology. `` Pathology and many others that generally concern life sciences as applied to medicine.
How Medical Biology Has Shaped Medicine? Last decades of XX century are characterized by intensive development of molecular biology, introduction of molecular technologies into practical medicine, pharmaceutics, ag-
8
Chapter 1. Introduction into the course of medical biology. Levels of organization and fundamental ... Pharmacogenomics Gene
mRNA
Peptide
Protein
Metabolism
genomics
transcriptomics
peptidomics
proteomics
metabolomics
MALDI-TOF spectroscopy
2D-gel electrophoresis SELDI
RNA microarrays DNA microarrays
Genetic medicine Harmonization Evidence-based medicine
H-NMR spectroscopy
Function polysystem monitoring
Methods for evaluating organs and systems functions
Predictive Medicine of the ХХІ century
Preventive Personalized
Fig. 1.1. Genomics gives new tools for medicine development. MALDI-TOF – matrix assisted laser desorption/ionization; SELDI – surface-enhanced laser desorption/ionization; H-NMR – protein nuclear magnetic resonance
riculture and other. The achievements of molecular biology and nanotechnology give the new conception of medicine, making it predictive, personalized and preventive (Fig. 1.1). `` Basing on modern biochemistry and biophysics data, medicine gets new more deep understanding of normal cellular processes and molecular mechanisms of human diseases. `` Achievements of medical genetics help to understand etiology of hereditary disorders, including the multifactorial ones. Multifactorial (complex) disorders develop as a result of complex interaction of hereditary predisposition and unfavorable environmental factors. It is the most common group of disorders in human population. Examples are arterial hypertension, bronchial asthma, diabetes mellitus and many others. Early detection of hereditary predisposition permits to prevent manifestation of the disease, postpone its onset, choose adequate therapy, and improve the life quality. `` Practical medicine uses new methods of diagnosis based on studying of DNA and RNA molecules. Molecular-genetic methods are used in medical genetics, infectious disorders, oncology, forensic medicine.
1.2. Levels of organization and fundamental characteristics of living matter
9
`` Modern biotechnology creates new medicines for treatment of many diseases. Human enzymes, hormones, and vaccines are produced by genetically modified bacteria, animals, and plants. It is possible to choose individual treatment regimen, based on the peculiarities of genetically determined metabolism of a patient. Modern biology also influences many aspects of science and everyday life. For example, molecular genetics studies stimulate development of evolution theory. It explains some evolutionary puzzles of anthropogenesis, demonstrates relationships between species and clarifies classification of living organisms. Synthetic biology (combines biology and engineering) permits to create artificial biological systems and redesign naturally existing ones. Modified bacteria can be used in diagnostics of diseases, detection of chemical agents, cleaning up environmental pollutants and have many other applications. Genetically modified plants are cultivated in many countris, giving an excellent crops and raw material for industry. However, ecologists are afraid of unpredictable influence of such organisms on the ecosystems. Our knowledge of proper nutrition, healthy life style, prolongation of life span is based on the achievements of medical biology. Practical work of the first year students in the field of medical biology begins with study of a chemical organization of a cell, its morphology and physiology, bases of genetics and reproduction, peculiarities of a special interaction between organisms – parasitism, later it continues into understanding of human ecology and biosphere with a glance on evolution.
1.2. Levels of organization and fundamental characteristics of living matter Living organisms are the opened self-regulated, self-renewing and self-reproduced systems composed of biological polymers – proteins and nucleic acids. All living organisms share several common principle characteristics: 1. Sensitivity or response to stimuli – organisms respond to changes in their environment. 2. Reproduction – organisms produce offspring similar to themselves. 3. Growth and development – organisms grow in size and change. 4. Metabolism – organisms carry out different chemical reactions, exchange energy and matter with outer environment (is an open system). 5. Homeostasis – organisms maintain a relatively constant internal environment. 6. Heredity and variation – organisms can transfer its characteristics to next generations and get new features. Living organisms are characterized by combination of these properties. The only one property is not enough to characterize an entity as alive.
10
Chapter 1. Introduction into the course of medical biology. Levels of organization and fundamental ...
Levels of organization of life All living organisms follow a hierarchy of organization in which simpler structures combine to form the more complex structures of the next level. This hierarchy can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. There is no difference between atoms of living organisms and nonliving matter. Atoms form molecules which are chemical structures consisting of at least two atoms held together by one or more chemical bonds. Set of molecules in living and nonliving entities is not the same. Dissimilarity became apparent starting from molecular level. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example
Biosphere
Organ
Tissues
Ecosystem
Population
System of organs
Cells
Fig. 1.2. Levels of organization of living matter
Organism
Molecules
1.2. Levels of organization and fundamental characteristics of living matter
11
of a macromolecule is deoxyribonucleic acid (DNA), which contains the instructions for the structure and functioning of all living organisms. So, the first level of organization of life is molecular-genetic level (Fig. 1.2). 1. Molecular-genetic level. Its elementary units are DNA, RNA and proteins. Elementary phenomena at this level are DNA replication, protein biosynthesis (gene expression). The processes at this level are studied by biochemistry, molecular biology, and molecular genetics. Mistakes at this level, like DNA mutations, cause single gene disorders. 2. Cellular level. Cell is the main structural and functional unit of life. A cell cycle and cell metabolism are elementary phenomena of the level. All main characteristics of living things are observed at this level. This level is studied by cytology and histology. Cellular pathology is underlying basis of any human disease manifestation. Failure of cell cycle regulation causes tumor growth. 3. Organism level. Its elementary structure is an organism, having organ systems. Elementary phenomenon of this level is a complex of physiological processes that provides organism functioning and its adaptation to environment. Human organism is studied at this level by histology, anatomy, physiology, medical genetics and pathology. Any disorder (started as aberrant gene expression and dysfunctions on the cellular level) eventually manifests at the level of organism. Practical medicine deals with organism of a patient. 4. Population and species level. Elementary unit of species is population. Important genetic characteristic of population is gene pool (set of genes in population). Elementary evolution processes (mutations, recombination, migration, waves of life, natural selection and others) are elementary phenomena. Demographic characteristics of a population reflect the problems of health state. The main laws of heredity manifest in populations. This level is studied by population genetics, medical statistics, evolution theory and ecology. 5. Ecosystem or biogeocenotic level. It is dynamic system of populations of different species (elementary unit) interacting with each other and the inanimate environment. Biochemical cycling of matter and energy flow and interaction of populations of different species are elementary phenomena. Infectious and parasitic dise ases are the result of such interactions (parasite and its host). Endemic disorders are specific for certain ecosystems. Main problems of this level are studied by ecology; epidemiology and parasitology deals with medical aspects. 6. Biosphere. Its elementary structure is ecosystem. Global cycling of matter and energy is elementary phenomenon. Modern problem that realizes at this level is increased anthropogenic influence (destruction of natural ecosystems, eradication of species, biosphere pollution). Biosphere is studied by ecology. Levels of organization of life reflect one of the general principles of philosophy: increasing complexity creates new properties that exceed the sum of the parts used to form the structure.
12
Chapter 1. Introduction into the course of medical biology. Levels of organization and fundamental ...
1.3. Chemical composition of the cell Matter is composed of a limited number of chemical elements. All living organisms are made of the same chemical substances that perform the same general tasks.
Chemical elements Chemical elements are substances that can not be broken down into any simpler chemical substances or converted into other substances by chemical reaction. There are 92 naturally occurring elements, each differing from the other in the number of protons and electrons in its atoms. Both living and inanimate matter is composed of same chemical elements, about 25 of which are essential to life. According to its content in an organism (i.e. human body), all elements are divided into several groups: `` “Organic” elements make up to 98 % of living matter as they form organic biomo lecules and are the components of many inorganic substances. These are Oxygen (O) – 65–75 %, carbon (C) – 15–18 %, hydrogen (H) – 8–10 % and nitrogen (N) – 1,5– 3 %. `` Major elements (macroelements). These are phosphorus (P), sulfur (S), chlorine (Cl), sodium (Na), potassium (K), calcium (Ca), magnesium (Mg) and iron (Fe). Major elements total make up about 1,9 % of living organism. `` Minor elements or trace elements (about 0,1 % in sum): iodine (I), manganese (Mn), zinc (Zn), copper (Cu), fluorine (F) and other. Concentration of each of them is less than 0.001 %, but they are important components of hormones, enzymes, vitamins and other biologically active molecules. Various inorganic elements are dietary “essential” and its deficiency causes different disorders. Role of some major and minor (trace) chemical elements is given in Table 1.1.
Chemical compounds Combination of chemical elements form chemical compounds. There are two principle types of chemical compounds present in cells: `` Inorganic compounds. `` Organic compounds.
Inorganic compounds Inorganic compounds in human body are water and inorganic salts. Water comprises 66–67 % of adult human body weight. Water has unique chemical and physical properties that allow it to sustain life. It is a polar molecule consisting of 2 positively charged hydrogen atoms (H+) and 1 negatively charged oxygen atom (O2–). Thus, water molecule attracts other water molecules, forming weak electrostatic hydrogen bonds between oppositely charged atoms (Fig. 1.3).
1.3. Chemical composition of the cell
13
Table 1.1. Role of some chemical elements in living organism Role
Medical aspects
Is a primary component of the skeletal system and teeth. Is required for muscle contraction and blood clotting, regulates heart rhythm
Calcium deficiency is hypocalcemia. Insufficient Ca leads to rickets in children and osteoporosis (weak bones) in adults
Phosphorus (P) 0,2–1,0 %
Is found in the bones and teeth, nucleic acids (DNA, RNA), ATP, phospholipids and other molecules
Phosphorus deficiency in blood is hypophosphatemia. It may cause rickets in children; improper balance of P and Ca may cause osteoporosis
Potassium (K)
0,15–0,4 %
An essential cation important for nerve transmission and muscle contraction. Regulates heart rhythm
Deficiency of potassium in blood is known as hypokalemia. It causes muscle weakness and irregular heart beat
Sulfur (S)
0,15–0,2 %
Is found in several amino acids (cysteine, methionine), vitamins (thiamine, biotine). Is important for detoxication (is a component of antioxidant glutathione)
Sulfur deficiency affects synthesis of connective tissue proteins, causes itching, brittleness of hair and nails
Sodium (Na)
0,02–0,15 %
Is a principal extracellular cation, important for nerve transmission
Deficiency in blood is hyponatre mia. The symptoms are weakness, headache, nausea and vomiting
Chlorine (Cl)
0,05–0,15 %
An essential anion important for regulation of osmotic pressure and acid-base balance. Is a component of gastric secretion (in the form of hydrochloric acid)
Deficiency in blood is hypochloremia. It leads to alkalosis (excess base), muscle weakness
Magnesium (Mg)
0,02–0,05 %
Is an important cofactor for many enzymatic reactions, including protein synthesis. Has an important impact on the balance of potassium and calcium
Deficiency leads to irritability and nervousness, muscle twitching or spasm, cramps, irregular heart beat
Iron (Fe)
0,01– 0,015 %
Is a component of hemoglobin in red blood cells and myoglobin in muscles (delivery and storage of oxygen). Is an important cofactor for many enzymatic reactions
Deficiency causes anemia
Element Calcium (Ca)
Weight percentage 0,04–2,0 %
14
Chapter 1. Introduction into the course of medical biology. Levels of organization and fundamental ...
Element
Weight percentage
Role
Medical aspects
Iodine (I)
0,0001– 0,014 %
Is necessary for synthesis of thyroid hormones
Deficiency in diet causes goiter (enlargement of thyroid gland). Hypothyroidism (insufficient production of thyroid hormones) causes weakness, depression, poor memory, weight gain
Fluorine (F)
0,0001– 0,0037 %
Is required for strength of teeth and bones
Deficiency leads to caries and weak bones
Hydrogen bonds give such properties as Н Н high cohesion of water molecules and high heat of vaporization. Cohesion is tendency of – molecules to stick together, that, for example, + Hydrogen helps water to move against the force of gravibonds Н ty in the vascular tissues in plants. Evaporative cooling prevents overheating in humans by – evaporation of sweat. Н + Polarity of water molecules explains also + Н Н ability of water to be a versatile solvent. – The importance of water in the cell: Н ` It is a medium for biochemical reactions Н and its participant. ` It helps cells to keep their size and shape Fig. 1.3. Water molecules with hydrogen bonds (turgid pressure). ` It is a solvent for many polar molecules. Water-soluble molecules are called hydrophilic, water-insoluble molecules are hydrophobic. ` Water helps in the maintenance of a stable internal environment within a living organism. The concentration of water and inorganic salts that dissolve in water is important in maintaining the osmotic balance between the blood and interstitial fluid. ` Water keeps temperature of cells from changing rapidly. ` It is the transport medium in the blood. ` It helps in lubrication. Inorganic salts mainly exist in form of ions (anions and cations). The most important are K+, Na+, Ca2+, Mg+, Cl–, HCO3–, HPO42–. Inorganic salts support osmotic pressure, form buffer system for maintaining of pH (CO32– and HCO3–; HPO42– and H2PO4–), activate enzymes and perform many other important functions. Anion HCO3– is a main form of CO2 transport in blood. Salts of Ca and P are important components of human bones.
1.3. Chemical composition of the cell
15
Organic compounds Organic compounds are complex molecules that have carbon chain as a molecular backbone. The main organic compounds in living organisms are carbohydrates, proteins, lipids and nucleic acids. Carbohydrates are organic substances with molecular formula Cn(H2O)n or Cn(H2O)m. Principles groups of carbohydrates are monosaccharides, oligosaccharides (disaccharides) and polysaccharides (Fig. 1.4). Monosaccharids or simple sugars are single molecules. They are categorized according to the number of carbon atoms. Pentose sugars (five-carbon sugars) ribose and deoxyribose are components of nucleic acids RNA and DNA. Hexose sugars (six-carbon sugars) glucose, fructose and galactose are involved in the production of energy. Disaccharides consist of two monosaccharide molecules joined by glycosidic bond. Examples are sucrose (table sugar), maltose and lactose (milk sugar). Polysaccharides are polymers formed by chains of monosaccharides (usually glucose). Well known examples in plants are starch and cellulose, in animals – glycogen. Carbohydrates are one of the main nutrients in our diet. The main functions of carbohydrates: `` Supply immediate energy for cell processes (monosaccharides, particularly glucose). `` Serve as a material for synthesis of other organic molecules. `` Store energy. Glycogen accumulates in liver and skeletal muscles and brakes down into glucose when required. Lipids are a group of hydrophobic molecules that include fats (neutral lipids), waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), phospholipids, and others. Some functions of lipids: `` Store large amounts of energy over long periods of time. Neutral lipids are stored in adipose tissue of skin.
Cellulose Source
Plant
Starch Amylose
Amylopectin
Plant
Plant
Diagram
Shape
Fig. 1.4. Polysaccharides that contain glucose as a monomer
Glycogen Animal
16
Chapter 1. Introduction into the course of medical biology. Levels of organization and fundamental ...
`` Act as an energy source. Complete oxidation of lipids releases more than twice energy produced by carbohydrates and proteins. `` Phospholipids play a major role in the structure of the cell membranes. `` Act as a source of metabolic water. `` Steroid hormones and fat-soluble vitamins are involved in regulation of different metabolic processes. Proteins are the polymers composed of 20 main types of amino acids. The amino acids are joined by peptide bonds into a polypeptide chain. A number of amino acids in polypeptide chain varies greatly. An average number of amino acids in human proteins is 447. Proteins have four levels of structural organization: primary, secondary, tertiary and quaternary. The primary structure is a polypeptide chain (linear order of amino acids). Secondary structure is of two types: alfa helix or beta sheet. Both variants are supported by hydrogen bonds. Tertiary structure is a globular-shaped molecule with a complex internal organization. It is held by different types of bonds, like ionic, hydrogen, disulfide ones and hydrophobic interactions between amino acids’ radicals. Tertiary conformation of the protein is of the utmost importance for its function, conformational changes contribute to various activities of the proteins. Quaternary structure is formed when protein is composed of several polypeptide chains (Fig. 1.5). Structural conformation of a protein depends on its primary structure. Sequence of amino acids is encoded in genes. Genetic (hereditary information) is realized by synthesis of proteins, and in fact it is an information about a set of proteins in organism. Proteins are building blocks of life. Cells generally contain a greater variety of proteins than any other type of macromolecules.
H3N+
alfa helix
C
COO–
A
B
D
beta sheet
Fig. 1.5. Levels of structural organization of proteins: A – primary structure; B – secondary structure; C – tertiary structure; D – quaternary structure
Tasks & questions
17
The main functions of proteins: `` They are structural components of cell and intercellular matter. `` Most of enzymes are proteins. They catalyze reactions of cellular metabolism, growth and repair. `` Some hormones, which control growth and metabolism, are proteins. `` Such proteins as antibodies and clotting factors provide defense of an organism. `` Recognition and signaling are provided by receptors of cell membrane. `` Proteins provide different kinds of movements. `` Transport of different molecules is provided by transmembrane proteins and blood plasma proteins. Nucleic acids are biopolymers composed of nucleotides. There are two types of nucleic acids: DNA and RNA. DNA is the largest biomolecule in a cell. Nucleic acids contain the genetic information and play a vital role in protein synthesis. More information about nucleic acids is given in Chapter 4. Most of human diseases are result of defects in biomolecules, failure of chemical reactions or biochemical pathways.
TASKS & QUESTIONS `` MULTIPLE CHOICE QUESTIONS (Choose one correct answer) 1. The mechanism by which organisms maintain the stability of internal environment is known as: A. Homeostasis D. Osmoregulation B. Normal health E. Blood circulation C. Structural adaptation 2. The ability to move is an example of: A. Homeostasis D. Adaptation B. Reproduction E. Response to stimuli C. Growth and development 3. The amount of sugar in our blood is always maintained 3.2 – 6.1 mmol/l. It is an example of: A. Homeostasis D. Adaptation B. Reproduction E. Response to stimuli C. Growth and development 4. The next higher level of biological organization above the organism is: C. Population-species E. Biosphere A. Molecular genetic B. Cellular D. Ecosystem 5. What level of life organization is studied by epidemiology and parasitology? B. Cellular A. Molecular genetic
18
Chapter 1. Introduction into the course of medical biology. Levels of organization and fundamental ...
C. Population and species E. Biosphere D. Ecosystem 6. One of the lipid functions is: A. Provide different kinds of movements B. Store large amount of energy over long period of time C. Contain genetic information D. Provide lubrication E. Play role of enzymes 7. Which of the following is a major element found in cell: A. Copper C. Sulfur E. Fluorine B. Zinc D. Iodine 8. Which of the following is a minor element found in cell: A. Oxygen C. Sulfur E. Sodium B. Carbon D. Zinc 9. The most prevalent compound in a living cell: A. Protein C. Water E. Polysaccharide B. Nucleic acid D. Lipid 10. The largest biomolecule in a living cell: A. Glycogen D. Deoxyribonucleic acid B. Protein E. Triglyceride C. Cholesterol 11. Amino acid is a structural component of: E. Fat A. Nucleic acid C. Sucrose B. Protein D. Starch `` FILL IN THE BLANKS: 1. Water is a polar molecule which contains ___ hydrogen and ___ oxygen atoms. 2. Organic elements are ________. 3. Calcium is primary component of __________. `` TRUE OR FALSE: 1. Oxygen is a micro element in cell. True False 2. Sulfur is a trace element in cell. True False 3. Carbohydrates supply immediate energy for cell processes. True False 4. Magnesium serves as a cofactor for enzymes. True False
Chapter 2. Classification of living organisms. Non-cellular and cellular organisms. Prokaryotic cell
According to modern point of view, there are non-cellular infectious particles and living organisms, which have cellular organization. Non-cellular infectious particles include viruses, viroids and prions. All cellular organisms are divided into prokaryotes and eukaryotes by the cell structure, peculiarities of gene expression and basic metabolic pathways. Prokaryotic cell lacks nucleus and membrane-enclosed organelles. There are two domains of prokaryotes – Archaea (Archaebacteria) and Bacteria (Eubacteria). Archaebacteria are bacteria of extreme environments, such as hot springs and salt lakes (the Dead Fungi Plantae Sea) or methanogenic Archaea. They have Ciliates unique properties and features, which difAnimalia fer them from other bacteria. Eubacteria is Ameboids the large group of bacteria, including pathProtists ogenic for humans. Flagellates The cells of eukaryotes contain nucleus. Eukaryotes There is one domain of eukaryotes, called Eukarya. It is divided into several kingdoms: the Protists (unicellular eukaryotes) and Archaebacteria three kingdoms of multicellular eukaryo Eubacteria Prokaryotes tes – Fungi, Plantae, and Animalia. The exact number of protists kingdoms is still under debate (Fig. 2.1). Fig. 2.1. Main systematic groups of cellular organisms
2.1. Non-cellular infectious particles. Viruses, viroids, prions Non-cellular infectious particles require host cell for existence. They include viruses, viroids and prions. Viruses. The first virus (tobacco mosaic virus – TMV) was discovered by the Russian scientist D. I. Ivanovsky in 1892. In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria.
20
Chapter 2. Classification of living organisms. Non-cellular and cellular organisms. Prokaryotic cell
For a long time it was believed that the viruses are living organisms because they have genetic material, reproduce, are able to adapt to the changing of environmental conditions and evolve trough natural selection. However, molecular biological studies revealed significant differences between viruses and cellular organisms. Thus, the genetic material of viruses is represented by only one type of nucleic acid (DNA or RNA). Viruses lack cell membrane, cytoplasm. They do not have their own protein synthesis system and other enzyme systems providing metabolism. They are incapable reproducing themselves and exist on the borderline between the living and the inanimate, non-biological world. Currently, viruses are considered to be infectious agents that can infect all types of living organisms including eukaryotes and prokaryotes. They are obligate intracellular parasites and reproduce by using the host cell to synthesize viral nucleic acids and proteins. The free viral particle is called virion (Fig. 2.2). Size of most viruses varies from 20 to 300 nm. Virion consists of nucleic acid (either DNA or RNA) and a protein coat or capsid. In some viruses, capsid is surrounded by membranous envelope. While in this form outside the host cell, the virus is metabolically inert. When it comes into contact with a host cell, a virus can insert its genetic Envelope Nucleocapsid material into the cell. An infected cell Capsomers produces more viral protein and geCapsid netic material instead of its usual prodNucleic ucts. Some viruses may remain dormant acid inside host cells for long periods, causNucleic ing no obvious change (a stage known acid as the lysogenic phase). But when a dorCapsid (composed of capsomers) mant virus is stimulated, it enters the lytic phase: new viruses are formed, selfassemble, and burst out of the host cell, Fig. 2.2. Structure of virion: naked virus (left) and envelkilling the cell and going on to infect oped (right) other cells. Role of Viruses: `` Viruses are the important factors of molecular evolution as they can transfer hereditary information between organisms of same or different species. `` Many viruses are pathogenic (can cause diseases). Diseases caused by viruses in humans include: flu, herpes, polio, rabies, chickenpox, rubella, mumps, Ebola, AIDS and many other (Fig. 2.3). Even some Fig. 2.3. Human immunodeficiency virus on the surface types of cancer have been linked of T-lymphocytes (microphotography)