Donald A. Windsor Norwich, NY
Hosts seem to be unable to repel parasites at the species level. If they were able, they would be parasite free, which they are not. In fact, some parasites have coevolved with their hosts in a continual arms race to the point where the hosts seem to be addicted to their parasites.
Why? Why is parasitism a property of life? Why is it so widespread? What is it that parasites do to maintain the susceptibility of their hosts? Why cannot hosts evolve to repel the parasites that harm them?
Predation is a property of life, but it is external to the prey, so the prey cannot do much about it. But parasitism is based on easy entry, so hosts do have a chance to thwart entry. Perhaps parasitism is a form of predation. But, even if it is, it is different enough to warrant special study.
Perhaps parasites put "cookies" on their hosts' genomes. Cookies are programming codes that one computer puts on another computer to recognize it. Perhaps parasites insert genetic, or epigenetic, material into their hosts' genome. These parasite cookies would allow free passage into or onto their hosts' bodies.
Sure, this is speculation on my part, because I have no data to support it. However, I prefer to regard it as a working hypothesis, one that can be tested by looking for parasite cookies.