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There are a surprising number of ways that criminals can use your personal information to commit fraud. Here are some of the weirdest:

1) They can steal your frequent flyer miles.

Whenever your credentials are valuable, thieves want them—even your frequent flyer credentials. In December, some 10,000 American Airlines and United accounts were hacked, and in at least two cases, cybercriminals actually booked free flights and upgrades using stolen perks, the Associated Press reported. The hackers had somehow gotten access to victims' login credentials. Happily, folks got their miles back, but this is just one good reason to change your passwords regularly and monitor your accounts.

2) They can steal your health insurance.

In a world of astronomical medical costs, insurance is very valuable. So it's no surprise that medical identity theft is on the rise. How it works: Identity thieves obtain medical services using your benefits, saddling you with their health records. It can take a lot of time and money to set things straight. You might not notice a problem until you get an Explanation of Benefits statement for services you didn't receive, so it's a good idea to actually read your insurance paperwork.

3) They can commit crimes in your name.

Here's a worst-case scenario: You get pulled over as part of a routine traffic stop, and you learn that there's a warrant out for your arrest for a crime you didn't commit. It can happen, if someone commits a crime and gives your name to the cops. Of course it's rare—but here's what the Privacy Rights Clearinghouse says you should do if it happens to you.

4) They can hack your company's chat system.

Has your boss told you to download HipChat or Slack for interoffice correspondence? Both have been hacked in the past two months. In February, HipChat reported that hackers made off with "names, usernames, email addresses, and encrypted passwords for a very small percentage (<2%) of our users." Similarly, Slack recently discovered that hackers broke into a database that stores usernames, email addresses, phone numbers, and Skype IDs for Slack users, and the messaging service notified a small group of people of other "suspicious activity." All you can do? Change your passwords frequently and set up two-factor authentication if available.

5) They can take over your social media accounts and impersonate you.

You've probably received a message from a friend that goes something like this: "I'm on vacation abroad, I lost my wallet, and now I can't get home. Will you wire me some money right away?" That's a scam called "social engineering." Identity thieves will hack your account, send messages to your friends, and try to ride on your reputation to trick people into sending you money. These scammers commonly use email and sometimes Facebook. To protect yourself, hone your BS radar—would your second cousin really ask you to wire money to Amsterdam without calling first?

6) They can steal your tax refund.

You need to file your taxes by April 15, but the IRS won't check your return against your employer's until July. That gives fraudsters a critical time window. With your Social Security number and name, an identity thief can file a fake tax return and collect the refund. You won't notice until your real return is rejected. Prevent this theft by filing early and trying these other tricks.

7) They can hold all of your computer files for ransom.

In perhaps the scariest online scam of our age, cybercriminals will use a "ransomware" virus to encrypt all of your computer files, then refuse to decrypt the files until you wire them thousands of dollars. (Yes, this happened on "The Good Wife.") The worst part? Right now, there's little you can do except pay, writes Alina Simone in the New York Times. Instead, prevention is key: Keep your security up to date, back up your files, and beware of suspicious links and attachments in emails.

8) They can open credit card accounts in your name.

When most people talk about identity theft, this is what they're really afraid of—not that someone will steal their credit card number, or their banking login, or the password to their email—but that someone will steal their Social Security number and start opening new accounts all over the place. If that happens, criminals can run up debt in your name, and you might not notice until your credit score tanks. So guard your Social Security number and check your free credit report three times a year for accounts you don't recognize.