Biology notes on viruses.

Notes pg. 455-461 Biologists first suspected the existence of viruses near the end of the nineteenth century. At that time, European scientists were seeking to identify the infectious agent responsible for tobacco mosaic disease, a disease that stunts the growth of tobacco plants and makes their leaves blotchy. One experiment used fine-pored porcelain filters to strain bacteria from sap extracted from infected tobacco plants. The pores in these filters were so small that bacteria could …

…host genomes. This is why viruses are almost always highly specific to the hosts they infect. Because viruses originated as fragments of bacterial and eukaryotic genomes, their great diversity is not surprising. Biologists think there are at least as many kinds of viruses as there are kinds of organisms. The number of viruses susceptible to humans is often, so the actual number of kinds of viruses may be much greater than the number of organisms.