Many people have heard about the dangers of identity theft, but now there is a devious twist to the crime.

Tens of thousands of Americans may already be victims of medical identity theft and not know it.

Pilot and business owner Joe Ryan thought it was a joke when he opened a letter from a collection agency.

The letter stated he owed more than $40,000 for a surgery that he never underwent.

"They said I had an operation and I went in and talked to them and I said, 'You want to see my body?' And they said 'No, no.' I said, 'I have no scars,'" said Ryan.

He discovered he was a victim of medical identity theft.

"It is when someone has taken your identity information, such as your name or your Social Security number, and they use it to get medical treatment in your name," said Pam Dixon, of World Privacy Forum.

The imposters simply provide fake insurance information, or none at all.

Dixon estimated nearly 250,000 Americans have already been victims of medical identity theft.

"It's extraordinary. You're looking at figures that can range upwards of $100,000," said Dixon.

But personal financers are only part of the problem.

Medical identity thieves can alter your health record, putting people in life-threatening situations.

"You can have a wrong blood type. You can have prescriptions on your record that you don't take," said Dixon.

Ryan added, "What happens if I drive home today, and I get in a wreck and I end up in a coma? They bring all those medical records, you know. Am I allergic to penicillin; is that the same blood type?”

The man Ryan believes stole his identity has since died, but the problems could haunt him for years.

"Now, he's dead. Am I dead? You know, will I ever get Social Security?" said Ryan.

More hospitals are now recognizing the crime and establishing programs to prevent it.

"Any patient coming in the door, we are asking them to provide identification for us. It could be a passport or a driver's license,” said hospital administrator Marie Whalen.

Whalen thinks the safeguards are working.

She said, "Two to three times a week somebody will say 'I don't have my ID' or 'I left it in the car' and then they never return."

How do you know if a medical identity thief has hit you?

If you receive a bill or an insurance claim in your name for medical services you never received, that is a red flag.

"Go ahead to wherever the bill was sent, call them up and ask them for a copy of your medical file. Work to change your records. Go to the insurance company; go to every place where you think your medical file has been,” said Dixon.

She warns that the process can be tough.

"Once your medical file is altered, it's almost impossible to get that information completely removed because it goes to so many places," added Dixon.

It is a nightmare Ryan is living every day.

The crime has deeply affected his life and his business.

"My credit started slowly going down and down. It's horrible. It's absolutely horrible," said Ryan.

While victims of financial identity theft have explicit legal rights to help clear their records, victims of medical identity theft currently do not have any blanket rights allowing people to correct errors in their medical files.