Spam! Spam! Spam! Spam! Spam! Carol asked why there's so much of it.

Why are there so many unwanted, tricky, manipulative, annoying, dangerous, and often illegal email messages going around? Because unscrupulous people can make a tidy profit mailing Spam. In 2010, Gmail spam expert Brad Taylor told Wired that "It costs $3,000 to rent a botnet and send out 100 million messages. It takes only 30 Viagra orders to pay for that."

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Botnets--illegal networks of infected computers controlled by people other than their legitimate owners--make an important part of the spam equation. Not only do they send out a great deal of spam, but their owners use spam to infect more computers, and thus grow their botnets.

These spammers can get your email address from all sorts of sources. They can search infected PCs for anything that looks like an address. They can read them off of incoming and outgoing email. Hackers can break into e-tailers' databases and steal addresses (and worse).

Read next: How to block and report iMessage spam

Posting your email address on a Web site, as I do for this blog, is asking for trouble. I get far more spam addressed to [email protected] than my other five email addresses put together. Luckily, mail to that address goes through two different spam filters before I get it.

So what can you do about it?

Be careful about sharing your email address. If you're uncomfortable with giving a site your address, either don't give it, and create a temporary, disposable one for that purpose.

Filter your email. Virtually all email clients--whether local or in the cloud--have spam filters these days. Use yours, and check it daily for false positives.

Be skeptical. Just because a message appears to come from a friend doesn't mean it has. If there's something odd about the message--if it doesn't read like something they would have written, or seems overly eager to have you click a link--don't trust it.

See also:

Why does everyone hate Appel right now?

Fixing a Safari-to-Mail disconnect

I didn't order that! What to do if your network provider account gets hacked