Why do fleas and lice exist?

The cooperation between bees or butterflies and flower plants naturally appeals to the imagination.

But the notion of “utility” is a typical human notion that puts man at the center of nature. However, this is a very relative term in biology. What is the use of man? We are very useful for fleas and lice! For unicellular parasites transmitted by fleas, such as plague bacilli, the flea is also useful because it can spread the bacillus. For monkeys, grooming each other is an important social activity that forges bonds within the group.

So in the end you can come up with a “use” for everything, but you don’t have to. In nature, something doesn’t have to be useful. All living beings are out to reproduce and to do so they use opportunities offered by their environment.