Yes it is becoming increasingly clear that some levels of parasitism are beneficial. The lack of parasites may be one of the reasons behind the increasing numbers of people with allergies and auto-immune diseases in developed countries. In countries with low to middle incomes, people who move from the countryside to large cities (to find work for example) also face a higher risk of developing allergies because they will eventually lose their parasites and will be exposed to less bacteria than if working e.g. on a farm. This has been called the ‘hygiene hypothesis’ of allergy.
One of the reasons is thought to be the fact that our immune system has evolved against a high background of parasite infection; the parasites will attempt to block or weaken the immune response ( in order to survive), and by doing so, they also inhibit diseases like allergy or auto-immune diseases. So if we lose the parasites, we are in a way sitting in a car driving really fast while throwing the brake out of the window…
Having said this, there are many parasites which have no benefits and only cause harm and death, so these are not worth having, and even the beneficial parasites (like human hookworms) are only beneficial in small amounts. If you have too many of them, the negative aspects prevail.
Some scientists are studying how parasites reduce or block the immune response because they believe that this may lead to better treatments for allergic diseases. Some even infect humans (in the context of strictly regulated clinical trials) with parasites to treat their allergies, or diseases such as Crohns’ disease, or multiple sclerosis.
Hello!! Parasites keep the balance of the world. Talking about parasites in the wild, they play a part in controlling population of one living things (plants or animals) in check and in balance with the whole system. For example, if the top predator are so successful, then the number of their prey can become the limiting factor, but parasites can come into play too, sometimes before the number of prey population got too low.
Inside the body, what Franco explained is one of my favourite area of parasite research. I always argue that there are more in the study of parasite biology than how to kill them.
Back to the wild again, some parasites are being used in agriculture. An example is a type of little worms, of about 1 mm long, which infect insects that feed on crops. They eat insects from the inside, produce lots more parasites, and burst out to infect the next insect. This give a nice alternative to using insecticide and the parasites do not infect human so it is quite safe too!
Fun fact: this parasite jump 10 times of its length to land on its insect host. 🙂