Question: how many parasites do people get in an average lifetime?

Claire Bourke answered on 20 Jun 2016:

Hi he who is not logged on,
that really depends on where someone lives and what their life style is like. In the UK we are fortunate to have access to lots of medicines, vaccines and sanitation that other parts of the world do not have access to, we are also lucky that our temperate climate means that it is not the natural habitat for mosquitoes, sandflies, kissing bugs or blackflies, all of which spread parasitic diseases, or tropical aquatic snails, which are needed by schistosome worms to survive and pass on infections. For those reasons, even though there are some parasites that affect the UK (e.g. pinworm), the average number of parasites we get in a year can be very low and a doctor would be very surprised if you turned up at the hospital with a tropical parasite! On the other hand, if you travel a lot to areas where there are more parasites, the average risk of parasitic infections would increase and, the risk of infection can be very high for people living in other parts of the world, particularly areas in the tropics and areas affected by poverty, low education about protecting yourself from becoming infected and poor sanitation.

In all areas of the world poor hygiene can increase the risk of infection because many parasites can be spread via contact with dirty water, soil, human and animal waste products and contaminated food. Often people who are already sick with one parasite are more likely to become infected with another because infection and illness weakens the immune system, which defends against infection.

Another thing to consider is that the risk of infection by parasites changes across a person’s lifetime. For many parasites, including malaria and some parasitic worms, the risk is highest in young children who tend to have poorer hygiene than adults, have immune systems that have not learned how to effectively kill parasites yet, and are still developing physically. As children grow into adults and gain good hygiene practices, a more experienced immune system and become fully developed physically, they are better able to defend themselves from infection so will be infected less often. In elderly people, the immune system tends to become weaker and so during old age there may be an increase in parasitic infections again. So, as you can imagine, in areas where there are lots of parasites young children can pick up lots of infections, then fewer with age and then more again when they become elderly. In the population that I used to work with in Zimbabwe, there were places were as many as 50% of people had a schistosome (parasitic worm) infection.

All of these factors (region of the world, hygiene, access to treatment, age…) as well as a range of others, means that the number of parasites that someone gets in their lifetime is very variable and calculating an average across everyone would hide this huge variety. Epidemiology is the study of the factors that explain the variation in infection levels and spread between people, communities and countries; by identifying risk factors for parasitic infections, we hope to find ways to reduce the harm caused by parasitic diseases worldwide.