Pathogens

Many diseases are most contagious in the prodromal period because of increased viral numbers and the lack of a clear disease state. This confers an advantage to the organism, because it maximizes the spread of disease before a quarantine can be effective. In reality, contagion of the disease can happen at any point in the illness process. Different diseases will be contagious at different times during the infection.

There are several types of infectious diseases. In an acute disease, the changes are relatively sudden, with active disease symptoms and short incubation periods. Influenza is generally an acute disease. Chronic diseases can last months or years and may never clear out completely. This involves a prolonged period of infectivity and communicability. Hepatitis C and Helicobacter pylori cause chronic infectious diseases. In latent diseases, the pathogen becomes dormant and is not transmissible. This is what’s seen with mononucleosis, caused by Epstein-Barr virus, herpes simplex viruses, and chickenpox viruses. The reactivation can happen many years later.

PATHOGENS

The understanding of pathogens was advanced by the work of Robert Koch, who developed Koch’s postulates. It should be noted that Koch’s postulates do not work in all cases of infectious diseases but are good general rules to follow. According to these postulates, an infectious disease must follow these rules:

1. The pathogen must be present in every person who has the disease but not in healthy persons. 2. The pathogen should be able to be isolated and grown in a pure culture. 3. A healthy person who becomes infected should develop the same symptoms as the sick person. 4. The pathogen should be isolated from the new host and must be the same as in the original host.

There are limitations to the postulates. Examples include Helicobacter pylori, which can be present in the healthy person and in the person with disease. In addition, not all healthy subjects can develop the disease when exposed. There are host factors that prevent getting the disease in certain resistant hosts. In addition, two different people

with an infection by the same pathogen might have different symptoms. Finally, not all organisms can be grown in pure cultures. This includes Rickettsia and Chlamydia, which must be grown with a host, and viruses, which are not easily cultured. Finally, because of ethical reasons, it is not possible to infect a person with HIV or other serious diseases.

A revised set of Koch’s postulates was developed during the 1980s. There are called molecular Koch’s postulates and uses the ability to detect a specific gene that is causative of a particular pathogenic disease. It relies on the ability to determine which gene is responsible for pathogenicity. It helps to explain why some organisms of the same species can be pathogenic, while others are not. It also helps to identify intracellular pathogens.

The revised postulates include the fact that the disease is only associated with the pathogenic form of a potential pathogen. In addition, inactivation of the offending gene diminishes pathogenicity and reactivation of the gene brings pathogenicity back.

An agent that can cause disease is referred as being pathogenic, while the degree of pathogenicity is called virulence. There are those organisms that have little virulence and those that are highly virulent. The symptoms of a low virulence organism will be less than those of a high virulence organism. An example of an organism that his highly virulent is Bacillus anthracis, which causes anthrax. This is a potentially lethal infection.

The virulence of an organism is quantified by detecting the median infectious dose or the median lethal dose, which are determined in animal models of the disease. The median infectious dose will be less than the median lethal dose. There is, however, a range of virulence that depends on the host’s age, immune status, environmental factors, host’s health, and pathogen-specific factors. Some organisms are pathogenic after just one organism has been passed to the new host but, if very few people die, it is not particularly virulent.

There are primary pathogens and opportunistic pathogens. The primary pathogen will cause disease in all persons, while the opportunistic pathogen will cause disease in hosts that are immunocompromised. These include cancer patients, young patient, old patients, and those with known immunodeficiency states. These types of pathogens can

be of any type or any virulence and depend mainly on host factors. Certain host environmental factors can lead to opportunistic infections. An example is Candida, which will cause disease depending on where it overgrows.

There are four stages of pathogenesis. There must be exposure, then adhesion or colonization, then invasion, and finally infection. Exposure relies on contact with the pathogen. There will be a specific portal of entry that includes the skin, parenteral routes, and mucous membranes. Mucous membranes include the anus, mouth, nose, eyes, or vagina. Parenteral portals of entry include needle injection. Skin can be a portal of entry if the skin is broken.

The most important portals of entry are the mucous membranes, which include the urinary tract, genital membranes, GI tract, and respiratory tract. Some mucous membranes are inside the body, while others are near the skin surface. Most organisms have a preferred portal of entry. Breaks in the skin and needle injection are both considered parenteral, while exposure to intact skin can also lead to some diseases.

The placenta can be an important portal of entry in the uterus during pregnancy. There are certain TORCH infections particularly known for this type of transmission. These include toxoplasmosis, listeriosis, syphilis, hepatitis B, chickenpox, HIV, Fifth disease, Rubella or German measles, cytomegalovirus, and herpes.

Adhesion depends on certain virulence factors called adhesion factors. Some pathogens have protein or glycoprotein adhesins, which bind to the host. Cilia, fimbriae, glycocalyces, capsules, membrane factors, hooks, barbs, spike proteins, and capsids can be adhesion factors. Protozoans will have barbs or hooks, while viruses have capsids and spike proteins that aid in adhesion. Biofilms have an extra-polymeric substance that enhances the attachment of the bacterial community. This is seen in Pseudomonas infections in the immunocompromised host.

Invasion proceeds after adhesion happens. The organism must be disseminated or may produce certain toxins or enzymes. Some virulence factors are specifically protective of the organism. There are certain virulence factors that determine what kind of damage is done to the host. This is true of H. pylori, which damages the mucin layer of the stomach lining. Intracellular pathogens will need to enter host cells to become invasive. Some