No real viruses exist, but malware (malicious software) does exist (apps you need to install yourself, but which then go on bugging other people by sending themselves ahead via Bluetooth or your address book & they can also trash local files).
Easiest to avoid: Do not install anything unsolicited sent to you, and do not install any pirated/cracked software (that may had embedded malware).
If you have gone and installed such malware anyway, then see, e.g., the F-Secure site (http://www.f-secure.com/) for tools & instructions to remove such.
Best antivirus software???
There's no 
Linear Mode
