Epidemiology

     Alright so Wiki crash course in what Epidemiology is. Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a society. Major areas of epidemiologic work include outbreak investigation, disease surveillance and screening (medicine), biomonitoring, and comparisons of treatment effects such as in clinical trials.

     So go figure the Greeks were the first to study epidemiology as well. The Greek physician Hippocrates is actually known as the father of epidemiology and is the first person known to have examined the relationships between the occurrence of disease and environmental influence. Apparently he thought up the terms endemic (for diseases usually found in some places but not in others) and epidemic (for disease that are seen at some times but not others). I think that it is astounding that thousands of years ago, people were already trying to figure out what was causing diseases (other than the gods being angry with them).

     So, Epidemiologists try to determine what factors are connected with diseases (risk factors), and what factors may protect people or animals against disease (protective factors). This area of science tries to understand possible causes of contagious diseases like smallpox, typhoid and polio among humans. I find it fascinating however that epidemiological studies can never prove causation. Epidemiology cannot prove that a specific risk factor actually causes the disease being studied, it is only able to show that this risk factor has a correlation with a higher incidence of disease in the population exposed to that risk factor. So how do they know if they have got the right pathogen (in the case of an infectious disease)? I know Koch’s postulates are supposed to help to make this link. For everyone who is unfamiliar Koch’s postulates are as follows:

1. The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
2. The microorganism must be isolated from a diseased organism and grown in pure culture.
3. The cultured microorganism should cause disease when introduced into a healthy organism.
4. The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

These were described quite a while ago and are now updated to look at genes and gene products, however these principles are still used. There are however some major problems with these postulates. Firstly, what about chronic infections, or people who remain asymptomatic? Well you could say to look for antibodies in the blood. However what if they had the disease previously and are now better, but because of memory, they still have antibodies? There is also the problem that some pathogens are unable to be cultured in a lab. Also what do you do in the cases (ie. in the gut or in the mouth) where there are many pathogens? So admittedly there are problems with the postulates but surely there are risks for not following the postulates as well.

One last thing before I finish with this session. I think it is interesting to see that chronic disease is replacing infectious disease as the most important causes of death.