It is difficult to ascertain the number of different viruses, as they increase in number every day. Although the number runs into the 1000s,it is important to note that very few existing viruses are widespread. A virus can enter a computer in a variety of ways. The precise symptoms of a virus will vary, depending on what the author of the virus has intended. Non-destructive viruses may display a textual message on your screen, or in the case of the Ambulance virus, a small graphic of an ambulance which crosses your screen.
More malicious viruses are often harder to discover.
By the time you notice things going wrong, the virus has already gone to work. There are no cast-iron guarantees to ensure that your computer will not become infected with a virus during its lifetime. However, you can take precautionary measures which will, if practised methodically, reduce the opportunity of viruses attacking your machine. Principally, should invest in an anti-virus program. There are many of these available, in both commercial and shareware flavours.