can cause genetic mutations and cancer. Aspergillus will also make gliotoxin that causes host cell destruction and it makes proteases, such as elastase and catalase, which will break down elastin and otherwise protect the fungus from the immune system. Protozoans can make adhesins and toxins; they also undergo antigenic variation and some can survive inside the cell’s phagocytic vesicles. Giardia lamblia has an adhesive disk that binds to mucosal cells in the intestines. It causes inflammation of the intestinal lining but doesn’t invade the cells directly. Antigenic variation can happen with organisms that cause malaria. The organism Trypanosoma brucei, which causes African sleeping sickness, makes a capsule and undergoes antigenic variation. Helminths, such as those that cause schistosomiasis, can penetrate intact skin by making elastin and other proteases. Helminths that are very large will evade the immune system and organisms like trichinella and other roundworms will have a cuticle surrounding them that evades the immune system. Still others express glycans on the cell surface that mimic host cells so they evade the immune system. Some will make proteases that degrade antibodies.
TRACKING INFECTIOUS DISEASES Infectious diseases are tracked by epidemiologists. This field of study looks at both disease transmission and disease etiologies. It studies populations of individuals and the specifics of the population that make it susceptible to a given disease. Individuals that have a disease are said to have morbidity with the morbidity rate being a certain number of diseased persons in a standard set of the population or a percentage of the population. Two important topics in epidemiology are incidence and prevalence. Prevalence is the number of people with the disease at a particular point in time, while the incidence is the number of new cases of a disease over a specified period of time. The lifetime prevalence is the number of people who get the disease in their lifetime. The prevalence of a chronic disease will always be higher than the incidence because the disease doesn’t go away. Mortality relates to death from a disease compared to a standard number in the population.
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