DALLAS (CBS 11 NEWS) – For Karrell Johnson of Dallas, a painful brush with a carbuncle started as a seemingly benign knot on his back.

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“I went out and played golf, “ said the UNT music instructor, “only nine holes because of the heat, but I wasn’t bothered with it at all.”

But, after a 2,500 mile road trip in the sweltering heat, Johnson says the ‘little knot’ turned into a nasty infection.  “It was just huge, and very infected, “ said Johnson.

Often called carbuncles or sebaceous cysts, doctors are seeing a rise in skin infections during this heatwave—as if the weather itself isn’t making us miserable enough.

“It can go on for days or even weeks, looking okay and then all of a sudden for whatever reason—sit for a long time, you get sweaty, you start exercising, and overnight it can turn south on us, “ said Dr. Jeff Goudreau, a Dallas internal medicine physician.

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Patients sometimes complain of fever, fatigue, or just a general discomfort.  Itching can occur before the carbuncle develops, and then the knot will turn painful and irritated with a white or yellow center.  As the infection worsens, the cyst will ‘weep’ and be filled with pus, and form a crust.

The infection is highly contagious and patients must take precautions to prevent the spread to other parts of the body , or to family members.  And be especially vigilant, doctors say, at the gym.

“You go to the gym, you do some reps on the weight machines and you get up without cleaning it, “ said Goudreau, “somebody else can lay down, sit down there, and have the same problem.”

Goudreau says the cysts must be drained—a process which is typically very painful—and are usually treated with antibiotics.  Otherwise healthy patients typically won’t suffer any long term health effects, he says… but, that’s not to say that the skin infections aren’t nasty and downright painful until they heal.

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Johnson headed to his doctor and is on the mend; but, admits he should’ve gone sooner. “I didn’t even want him to touch it, it was so sore.”