Question: what exactly is a virus .how do you get diseases?

Sharon Sneddon answered on 17 Jun 2010:

Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist as a “protein coat “, sometimes enclosed within a membrane. While in this form outside the cell, the virus is inactive. When it comes into contact with a host cell, a virus can insert its genetic material into its host, either DNA or RNA and it takes over the host’s functions. An infected cell produces more viral protein and genetic material instead of its usual products. When a virus is stimulated, it enters the lytic phase: new viruses are formed, and burst out of the host cell, killing the cell and going on to infect other cells.
Viruses cause a number of diseases. In humans, smallpox, the common cold, chickenpox, flu, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer — though definitely not all — have been linked to viruses.