Viruses circulate from one computer to another, often via diskettes. 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 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.
If you have files you can't afford to lose, make sure you have more than one copy of them. If using Windows or Win95, use the option to make a System Disk in File Manager or Explorer, as appropriate. They may come in handy if you can't access the hard disk, or it won't boot up.