It's true that some viruses cause strange things to happen. However, legitimate software can also cause these effects. And while some viruses are very obvious, displaying messages, or even playing musical tunes, many give no sign of their presence. So it's important not to assume your computer is infected, just because strange things happen. While the risk is relatively small, it is growing daily.
Viruses circulate from one computer to another, often via diskettes or via the Internet.
If you're lucky, you'll never encounter a virus, but one could be concealed in the next file you download, or on the next diskette you receive. Downloaded programs from the Internet can be infected, and viruses can travel among networked PCs. If you're concerned about the virus threat, the time to obtain anti-virus software is before you get a virus. It's much better to prevent a virus infection than to have to deal with one. Viruses are a threat, but data loss can also occur from other factors.