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Asked by ILuvScience to Claire, Franco, Koi, Linda, Mark on 13 Jun 2016.
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Mark Booth answered on 13 Jun 2016:
Hi Iluvscience – parasites have evolved to take advantage of humans in the same way that humans have evolved to take advantage of the earth – they need resources to live and help propagate.
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Franco Falcone answered on 13 Jun 2016:
Hello ILuvScience
parasites depend on their hosts (in our case: humans) for their surviveal. This is a result of evolution. For example, ticks and mosquitoes (which are called exoparasites, because they remain outside the human body) will need to feed on blood because they need this for the development of their eggs. In fact it’s only female mosquitoes that feed on blood (and can transmit malaria in the process), male mosquitoes are vegetarian, they feed on flowers, a bit like bees.
Successful parasites have evolved various ways of evading the immune response, in other words they ‘know’ how to to get killed. But every now and then, they can get into the wrong host, and there they can get killed.For example some worms infect dogs but can also get into humans, but cannot develop there, and will eventually die as a result (Toxocara canis for example)
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Arporn Wangwiwatsin answered on 13 Jun 2016:
Hi ILuvScience, I’d say because they have evolved to be able to take advantages of resources from inside the human body without getting killed.
In every places to live, there are challenges and there are benefits. Inside human body, the challenges can be our immune system that work to keep us from infection, and entering the body might be hard because we have skin as a barrier, and also in gut we have lots of digestive juice that digest our food (so might as well digest parasites?!). However, if the parasites have evolved enough tactics to get away from these challenges then they can benefit from being inside our body, which includes things like…. it’s nice and warm in there, there are plenty of food supplies, and we take them places making it easier for them to spread.
THere are also parasites that live on human body too, like head lice… and also let’s think about some non-parasites too; some animals can live in a very dry and hot place because they have evolved to cope with that and get the benefit because …not many other managed to do so! (Let’s stop here before we moved into the Ecology Zone! :P)
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Linda Anagu answered on 13 Jun 2016:
@ ILuvScience, they survive better inside the human body and have developed ways to ensure that they can enter the human body and cells.
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Claire Bourke answered on 13 Jun 2016:
I agree with the other scientists. Parasites can use the resources of the person or animal that they infect (food, warmth, protection…etc) to enhance their survival. Not all parasites need to live inside a host, some can live on their surface (e.g. Ticks and Mosquitos can feed on blood and exist in the outside environment too), but living inside the body is one way of avoiding the challenges posed by outside world. Some parasites have stages of their lives where they live outside the human body and others where they live inside. In the case of schistosomes, the parasitic worms that I study, they have a stage the lives inside snails, then a stage that lives by itself in water, then a stage that lives inside the human body and then another free-living stage. All these stages show how, even for the same parasite, there are benefits to being inside and outside a body! Imagine how complicated it would be to keep switching environments like that!
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