That's what Kaspersky is for:
Advanced anti-virus protection for your Mac. Stops malware before it can be passed from your Mac onto PCs and other systems on your network. Small, frequent updates. Outstanding performance. And a familiar Mac-type interface.
There are no (NO, NADA, NJENTE, NULL, ZERO) viruses for Mac OS X, so downloading Kaspersky would only be to prevent spreading a Windows virus that has been sent to you and which you send to another person.
If you feel obliged to protect other people's computer, go for it.
But in the end, the person owning the computer is responsible for the security, not you.