We all know that many diseases are caused by viruses, bacteria, or parasites. For instance, HIV is caused by the HIV virus; tuberculosis by the bacterium Mycobacterium tuberculosis; and malaria by the protozoan parasite Plasmodium falciparum and others of the same genus. Yet not everyone who’s exposed to a pathogen gets the associated disease. For instance, only 3 out of 1000 people who get stabbed with an HIV-contaminated needle will actually develop HIV. But why is it that some people get these diseases and others don’t? And is there any way we can use this information to protect more people?
Epidemiology 101
Welcome to the field of epidemiology, the study of the distribution and causes of disease. Commonly called the science of public health, epidemiology examines the many different factors that contribute to disease to try to find out how we can keep people healthier. There are a lot of different factors that impact health – not only exposure to pathogens, but also diet, genes, age, socioeconomic status, local water quality, population density, and others. Epidemiologists look at who gets sick, where, and when to try to figure out what factors are contributing to the illness and of those, which we can do something about.
Generally speaking, an epidemiological study looks at two groups of people: those who got sick and those who didn’t. The researchers then try to find out everything about those people – where they work, what they eat, what kind of paint they have on their house, how many pets they own – and then do statistical analyses to see if any of the factors they asked about are associated with the disease. These studies are often time-consuming and may require very large cohorts of subjects to be effective, but their results can be crucially important to the field of public health.
Epidemiology is the reason we know that smoking causes cancer and lead causes brain damage. In addition to looking at environmental toxins, many epidemiologists study chronic diseases, such as cancer and heart disease. These diseases can obviously be caused by many different factors, including exposure to various toxins, diet, genetic factors, and others. There are also a growing number of epidemiologists who study the factors that increase the likelihood of contracting an infectious disease. Some of these factors are pretty obvious too – getting a mosquito bite in a malaria-endemic country, for instance, or sharing needles with an HIV-infected person. But some of the risk factors for infectious diseases are a bit more surprising.
An example – leishmaniasis
One interesting case is that of visceral leishmaniasis in East Africa. Leishmaniasis, one of a group of diseases known as “neglected tropical diseases” due to the lack of press and funding it receives, is caused by a protozoan parasite and is transmitted by the bite of the sand fly. Visceral leishmaniasis is the most severe form of the disease, in which the parasite migrates to the vital organs, and it kills an estimated 60,000 people per year. Rates of visceral leishmaniasis have risen dramatically in East Africa over the last 20 years [1], and recent studies have attempted to figure out why.
A lesion caused by cutaneous leishmaniasis – a.k.a. leishmaniasis on your skin. Visceral leishmaniasis is like this, except on your liver.
Some of the reasons are not surprising – increased incidence of HIV, for instance, has contributed to the increased incidence of leishmaniasis just as it has contributed to an increased incidence of many other diseases [1]. But other reasons are more startling. For instance, you might think that if you owned cattle or other animals, treating them with insecticides would be a good preventative measure against leishmaniasis. The insecticides would keep the sand flies away from the cattle and thus away from you; with no sand flies to transmit the disease, no leishmaniasis. Makes sense, right?
Wrong. It turns out that treating your animals with insecticides actually increases your risk of contracting leishmaniasis [1]. The idea seems to be that if the sand flies can’t bite your animals, they’re more likely to bite you instead!
Another example – umbilical cords and mustard oil
Epidemiology also examines whether different treatments for diseases are effective or not. In southern Nepal, for instance, mustard oil is commonly applied to the cut umbilical cords of babies in an attempt to prevent infection. It turns out, however, that applying mustard oil is actually associated with an increased chance of umbilical cord infection – whereas applying mud or ash to the umbilical cord has no associated increased risk [2]. Of course, this doesn’t mean that applying ash to your baby’s umbilical cord is a good idea. Actually, in this particular study, a lot more people applied mustard oil than applied mud or ash, so it was just easier for the researchers to tell what the effect of mustard oil was [2].
Although mustard oil appeared to increase the risk of umbilical cord infections, the researchers in this study also found several techniques for decreasing the risk of infections. The more the mother and anyone else around the baby washed their hands, for instance, the lower the chance of infection [2]. In addition, the study found that keeping the baby warm, either through skin-skin contact or by giving the baby hat – also decreased the risk of infection [2]. These findings provide simple, easy-to-implement ways to keep babies safer and healthier.
Conclusion
Although some factors – such as poverty, malnutrition, and immunosuppression – are known to increase the risk of pretty much every disease, some diseases have surprising associations with factors that seem innocuous, or even with things that seem like they’d be good for you. It’s only through the careful study of the association between behaviors, exposures, and disease that we can determine what the risk factors for a given disease are. Then it’s time to move away from studies and into implementation, and use this information to devise better strategies for treating and preventing diseases.
References
1. Kolaczinski, J. H., R. Reithinger, D. T. Worku, A. Ocheng, J. Kasimiro, N. Kabatereine, and S. Brooker. 2008. Risk factors of visceral leishmaniasis in East Africa: a case-control study in Pokot territory of Kenya and Uganda. International Journal of Epidemiology 37(2): 344-52.
2. Mullany, L. C., G. L. Darmstadt, J. Katz, S. K. Khatry, S. C. LeClerq, R. K. Adhikari, and J. M. Tielsch. 2006. Risk Factors for Umbilical Cord Infection among Newborns of Southern Nepal. American Journal of Epidemiology 165(2): 203-11.
This post was written by lucy.mcnamara
