Email is one of the most common ways to infect a computer. It isn't the email itself, but what the email may contain.
It could be a 'phish'. This is an email that supposedly comes from your bank, credit card company or any other financial institution. PayPal and eBay have been also used for phishing emails. The way these emails work is by intimidation or scare tactics. The email will say that they have lost your details and have a link for you to click to log in and 'update' your information. If you don't update your information, your account may be closed or be unavailable. What the link will do is take you to a site that will just steal your credentials if you are foolish enough to give all of your login and/or financial details.
There are the emails from scammers. These are getting more sophisticated than the original Nigerian scam, but a savvy user can still recognize they are scams. Scammers may be scanning Facebook or using a compromised email account and finding information to try to convince a user that someone in their family needs help/been mugged. See Friend Stranded in Foreign Country
It could also be an email from a spammer who is trying to verify your email address is valid. If you reply to a spam email, it lets the spammer know the address is good and now your email address is on a list that can be sold to other spammers and soon your inbox is full of spam!
And of course there are the attachments that come in email.
They could be harmless, but they could also contain virus,
worms, keyloggers or any one of the myriad uglies that can wreck
your computer, steal your identity or wipe out your bank
account. And the sad thing is, you may think the attachment is
coming from a friend when in reality it is a 'spoofed' email
from who knows where!
So what can you do about all of these 'evil emails'? Don't open attachments unless you are expecting them. Don't click on email links unless you know where the link will take you. Delete unopened emails from strangers! And remember, your bank, your credit card company and any other financial site will never ever send you an email asking you to update your information! And keep your security scanner updated!