"You won!"

"Short on cash?"

Every day, about 45 million spam text messages are sent to North American cellphones. If you don't have a text message plan, you'll pay around 20 cents for each one you receive.

Sign up for the AARP Money Newsletter.

But the real cost comes if you respond to those micro messages about such things as free gift cards, cheap mortgages and meds, and urgent "account problems" at your bank.

That's because at least 70 percent of all cellphone text spam is designed to defraud you in some way, according to a study by Cloudmark, a company that makes anti-spam software. In contrast, only about 10 percent of spam arriving by email is sent with that intent.

Spam in text messages may try to guide you to shady websites that will install "malware" on your phone to vacuum up all the personal data stored in it. Or the messages may urge you to dial a phone number where your personal and financial information is solicited. Whatever you provide is either used to steal your identity or sold to third parties who'll send you yet more spam texts.

In 2012, cellphone spam — often called "smishing " — has tripled compared with 2011 levels, while email-delivered spam continues to drop, following an 82 percent decrease last year.

Learn how to avoid being scammed by a simple text message. — C.J. Burton/Corbis

Why? For scammers, cellphone text messages are fast, cheap, easy and effective. The thieves use inexpensive, hard-to-track prepaid phones to transmit myriad text come-ons. When word spreads of the spam du jour, the scammers simply toss the phones they're using and buy new ones.

What's more, cellphone users are three times more likely than computer users to respond to spam. "Because you always have your cellphone with you, answering it becomes more of an impulse reaction," says Cloudmark security researcher Mary Landesman.

Next: The best ways to avoid a text message scam. »