It's tough enough to deal with just one chronic disease, never mind being diagnosed with another, but it happens more than you think. Researchers are now learning about shadow diseases and why they seem to travel in pairs.

Sam Jonas has psoriasis, which leaves scaly patches on his skin

It can be very embarrassing, he said.

In 2006, Jonas also had a heart attack. He didn't know the two conditions are linked.

(I was) very surprised. I had no idea, said Jonas.

Dr. David Ancona, cardiologist at Memorial Hospital West in Pembroke Pines, Florida, says psoriasis doubles the risk for heart attack.

There are diseases that exist that, for some reason or another, seem to follow one another like a shadow, said Ancona.

Women with endometriosis are 62 percent more likely to develop melanoma.

It's becoming more clear to us that these diseases don't exist in isolation, Ancona.

What's less clear is why.

Certain diseases might be genetically linked, said Ancona.

Having just one migraine a month gives you a 50 percent higher risk of a heart attack. Having one a week gives you four times the risk of a stroke. And having three of the five traits linked to metabolic syndrome ups the risk of kidney stones.

The benefit of the patient knowing that they could be at risk for another disease would be that they'd be evaluated for that other disease, said Ancona.

Sam Jonas wishes he had been aware of his risk.

Had I known that at that time, I probably would have gotten a stress test much earlier, said Jonas.

That step might have saved his heart. Instead, he'll take daily drugs to treat both conditions.

Having either psoriasis or insomnia increases the chance of developing diabetes. And research shows nearly 50 percent of people with asthma also have a psychological condition like depression.

Again, just because you have one chronic disease doesn't mean you'll get a second one, but knowing the risk can help you take steps to prevent the other.

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