"Malware" is a generic name for malicious software, and a "virus" is a specific type of malware.
"Viruses" are rare on OS X, where a different sort known as "trojan horse" is more common.
A virus generally requires no user access to install, where a "trojan horse" is something the user downloads and runs.
Now for the questions...
How do you know you're infected by malware here? What site or tool or package or person is telling you that?
Are you downloading and installing random software, including downloading and installing video players, or authorizing Apple.com certificates or such?
Are you current on your OS X patches, and your Java patches?