Why are there pathogens and are they useful?

Answer

Dear Chris,

Why should bacteria or viruses be useful to humans? It doesn’t work that way in nature – every living thing has only one goal, which is to reproduce itself. They can use other living creatures, for example a lion eats an antelope and an antelope eats grass. In this sense, grass is useful to the antelope, and the antelope is useful to the lion. But no living thing has evolved to be of use to another.

To come back to the pathogenic bacteria or viruses: humans are useful to them. Because they can live and reproduce in our body, and at the expense of it. Humans are therefore useful for those bacteria and viruses, and not the other way around.