We have shrunk the jungles, and think we have mastered all wild things, but those shrunken forests conceal the carriers of viruses which could wreak terrible destruction. There is a theory that some viruses are not just harmless to their hosts, but may even be beneficial to them because they cause deadly disease in their enemies - such as man. We still do not know the animal or insect which carries the Ebola virus, and one of the reasons could be that the virus has killed the few who have disturbed the animal's habitat. The closer man gets to the last wild strongholds, the closer he may be getting to the ultimate predator. Imagine a virus with the effect and latency period of HIV that could be spread by a sneeze.
Viruses are hard to describe.
As long as it isn't inside a computer's operating system, it's harmless, dormant, abstract. Once the code is in a computer, however, it does three things. It acquires a host, it begins to make the computer run wrongly, and it can reproduce itself to spread to another computer. Biological viruses are similar. Outside living cells, they're not much more than a piece of information, a scrap of nucleic acid in an envelope of protein.