Virus

 When I was in college at USF, we learned about infectious agents like viruses and how they infected the host cells and used the DNA and RNA of the host to make new virus bodies. They would make copies and these copies would break out and infect other cells over and over until the hosts contained millions of copies. But not all the copies were exact copies, some of the copies contained differences and this is one way we get the different strains of viruses. The same Influenza virus seems to come back every year but there are also different strains too. I even read that the virus from 1918 still shows up every year. If the virus strain is very deadly and kills the host quickly, then this strain is not spread as much, since the host does not live very long. So the most damaging strains appear to disappear or have been modified into a less damaging strain. And it seems like viruses has a greater affect on the population that already has health issues, like heart disease and other life...