As a parasitologist, I am often asked about the impact of parasites on man. Major killers such as malaria, which claims the lives of more than a million of the 300 million afflicted each year, grab our attention. Parasites, however, are everywhere. Nearly everyone in the developing world carries worms that don't always kill them, but leach away already scarce food and undermine their victims' health in other ways.
Even in the sanitised west, many of us have had pinworms or head lice. Around a third of us carry toxoplasma parasites, single-celled bugs that can contaminate meat and get transmitted in cat poo.
The word "parasite" derives from Greek and means "one who feeds at the table of another". Contrary to popular opinion, parasites aren't degenerate regressions on the basic plan of life. They are dominant.
Most species host dozens of different parasites, all of which have evolved sophisticated ways to facilitate existence within a living habitat. Take Cymothoa exigua, a crustacean whose larvae enter the gills of the spotted rose snapper. They suck blood from the fish's tongue until it shrivels and dies. The parasite then fixes itself to the tongue muscle and stays there for the remainder of its life, acting like the lost tongue and enjoying first refusal on any food that comes into the fish's mouth.