Most times you get a "virus" its actually a trojan.. Sometimes you can download a program that seems to work as it is supposed to, but it also contains coding to do other things. Trojans, and viruses can be downloaded in emails, and thrugh websites. It would be a very simple task to write a virus or trojan into simple HTML or java script that is run on most websites. Even an animated gif file can contain a trojan..
To say a mac doesnt get viruses is not exactly true, and I get really tired of hearing that, the fact is that more users online use Windows based PC's and it it the Windows OS that makes the computer venerable, no matter if it is a PC, or a MAC. Hackers havent really spent alot of time creating malware for the apple OS.. Probably because there arent alot of business based systems using them. and we all know that most malware out there is created to fight the man...
Anyway there are several ways to protect yourself from these programs, besides keeping your antivirus software up to date, you need to monitor your regestry, and watch for any unintentional changes.. watch for slowed computer speeds...
sometimes you will see returned email that may have bounced, and you didnt even send it. sometimes this can be a sign of a virus, but often times it is not. Spammers copy email adresses, and use them as return emails fro their crap, so they will send an email using your email as a return adress, and you will get all the replies, and failure notices.
41 viruses is alot, you shouldnt see that many if you are monitoring your system.. In this day and age we all need to be more careful.