Reply #19: co-evolution

[northernlights]

Generally, viruses and bacteria co-evolve with their hosts. I call it "survival of the less fit."

The most virulent pathogens kill their hosts too quickly and efficiently to spread, so die out themselves. That is why ebola outbreaks tend to be limited. The victims don't live long enough to spread the disease very far.

However, less virulent pathogens don't kill off their hosts, and both pathogen and host evolve to a degree of equilibrium. The host develops some immunity to the pathogen, but low levels of pathogen are overlooked by the immune system. They aren't present in large enough quantities to induce an immune response or to significantly impair host functioning. When their numbers rise to a point where the immune system notices, it beats them back to low levels. If the host becomes overstressed, however, opportunistic infection can take off. Classic example is herpes simplex, that swings between "remission" and "active infection."

In some cases, they can evolve to a symbiotic relationship or even become part of the host (as is suspected with mitochondria).