The truth is that up to 25% of the US personal computers out there are infected with viruses and/or worms or trojans. I think that figure of 25% is low, and if you add in the security-compromising, computer-slowing spyware, the figure jumps to more than 70%. People either do not know how to properly set up anti-virus software, don't know that it's necessary, or do things with their computers that break security.
Like...not having any anti-virus/anti-spyware software installed in the first place.
Like...having anti-virus software from 2004, with an expired subscription, that does not protect against current threats.
Like...surfing to some of the infected web-sites their that will try to infect your system -- up to 33% of all web-sites.
Like...opening that email attachment because it says you just won the lottery, or because it's from someone whose name you think you might recognize.
If you want to take a look at what nasties areout there and how it might be affecting you. check out the Panda Security site map page, for cybercrime software and classic malware.
There are 2 parts to software security on PCs: anti-virus software, and firewall software. Windows XP and Vista come with firewall software. Do make sure that it, or a commercial replacement is enabled and working. Windows will warn you if you do not have firewall protection. You also need anti-virus software, and by that I mean to include anti-spyware, anti-pop-up and other bad stuff installed. Such software is available commercially, and also freely, and can be downloaded over the internet. Windows does not come with a complete anti-virus software package.
Those of you with Vista already have some software protection in Windows Defender, which XP users can download and install.
There is also hardware security for a PC network, which, at its most basic, can be taken care of by your router, and which will protect you in some ways like your software firewall.
How do you know you are infected or have a problem? Well, most of the time you already KNOW you have a PROBLEM, because your computer is acting funny in some way, that does not appear to be hardware issue.
If you are having a problem with a slow computer, lots of email messages talking about "Undelivered Mail" from recipients you don't know, you may have a virus or multiple virus infection. At worst, your system may have been"taken over" by a "botnet" and remotely controlled to send spam or steal credit card info, perhaps even your own.
What can you do about it?
Well, first, to prevent the probem(s), there are 3 basic things you should do, or have done already:
1) Use a good, cheap NAT router, wireless or otherwise, that blocks certain types of attacks, such as the $60 (street price) Linksys WRT54G, which will also hide your computer(s) from outside systems.
2)Install a good, cheap Anti-Virus software on your PCs (how does FREE sound?), such as AVAST! Home Edition, which does a very good job, and which is free to home users.
3) Don't do dumb things. Don't open software that appears in a pop-up on your screen offering to Speed Up your system, or clean bad software from your drive, or...whatever. Don't open email from businesses or people you don't know. And don't, don't, don't, open attachments in these messages.
Also, if you think your computer is infected, for goodness' sake (!), do not try to use it do do stuff, like ... send email, which may infect other machines, or make an on-line transaction, which may compromise your credit and banking info, if it hasn't been already!
Once your computer is infected, you have to clean it first. If your computer will still run, and you can download and install AVAST!, you can schedule a boot-time scan, so, after you have the software installed, you reboot the PC and the anti-virus software will scan and quarantine the malware. After the system comes back up, you will have to opportunity to delete the quarantined files.
If you can't boot your system, there are commercial products that will load an AV (anti-virus) product from a CD and scan your hard drives for malware. One of the most well known is Norton Anti-Virus .
Sadly, sometimes the Windows operating sytem files are so completely compromised, that the infected PC will not boot. If that is so, you can still save whatever you have on that PCs drive, by booting from a new drive with the old drive attached to the PC as a secondary drive, and then cleaning that drive.
Now that may mean opening up the box and getting into the machine, and is the subject for a whole 'nother post.
No, they don't spread to humans...