How difficult is “parasitic” life?

  It seems that the parasite does not need to withstand the wind and sun, and does not need to hide in order to avoid predators, as long as it “lays flat” in place, it can obtain a continuous supply of food. But the life of parasites is not as easy as imagined, and it also requires great efforts and sacrifices.
  The first problem that parasites encounter is that the living environment limits their growth, and they can only adapt to the environment by changing their shape and size. For example, intestinal parasitic nematodes and tapeworms mostly evolve into threads or ribbons to adapt to the narrow and long intestinal cavity; a parasite called crocodile lice usually parasitizes in the mouth of fish, and their body shape is also similar to that of fish. Oral fit.
  Secondly, parasites living in different parts have to change their own physiological structure to adapt to the host’s organ activities. For example, in order to prevent the peristaltic intestinal tract from expulsing itself, parasites living in the digestive tract of the host need to evolve powerful locomotor organs or equipment such as suckers and small hooks to adsorb on the sticky surface of the digestive tract full of digestive juices.
  The biggest challenge comes from the host’s immune system. As a foreign object, staying in the host’s body for a long time will inevitably cause the host’s immune response. In order not to be attacked by the host’s immune system, some parasites will disguise themselves as the host’s “self” components, such as African trypanosomes, Schistosoma mansoni, etc. . Of course, not all parasites have this “superpower”. Most parasites will form a state of balance with the host, neither causing the death of the host, nor will the host react violently to eliminate the parasite .
  The last problem to be solved is the problem of reproduction. It is precisely because parasites face many difficulties in the host body, in order to maintain the continuation of the population, parasites need to evolve super reproductive ability. Each female roundworm lays about 200,000 eggs per day, and a female W. bancrofti can produce millions of larvae in her lifetime, so that the population can be maintained.