Sarah Larimore, 7, can't stay in 80-degree weather for longer than 20 minutes.

(ABC News Radio)

Sarah Larimore, 7, didn't always like summer.

She suffers from a rare genetic metabolic disorder called cystinosis, which prevents her from sweating. She lives in South Carolina, and can't stay in 80-degree weather for longer than 20 minutes, so during the summer she used to stay cooped up inside, her mother, Katie Larimore, told ABC News.

Last summer, her family built a swimming pool in their backyard.

"It was her first real, live outside summer," her mother said. "She just stayed in the water for three months."

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Sarah is one of at least 25 million Americans who suffer from rare conditions, according to the National Institutes of Health, which recognizes Thursday as Rare Disease Day. The last day of February has been promoted since 2009 by the National Organization for Rare Disorders (NORD) to increase awareness about the common needs of people with illnesses that not many others have.

People with a rare illness -- defined as affecting fewer than 2,000 people -- share common challenges: They have trouble getting an accurate diagnosis, finding treatment, and convincing insurance companies to pay for care, said Mary Dunkle, a NORD spokeswoman.

"Even though each disease is different, the challenges of living with a disease that's rare is pretty consistent," she said.

Of the 6,000 to 7,000 rare diseases recorded in the U.S., treatment options exist for only a few hundred of them, Dunkle said. Treatment is more widely available than it was 30 years ago when NORD was founded, but there's a long way to go, the spokeswoman said.

Federal aid for people with rare illnesses is precarious, and it's often tough to persuade a pharmaceutical company to invest in treatment for a rare disease when there isn't a big payoff, Dunkle said.

Sarah, who loves reading and fashion as well as swimming, shares her diagnosis with about 500 Americans, according to the Cystinosis Research Network. Cystinosis causes an amino acid, cystine, to accumulate in every cell of the body, damaging organs and blood cells. An estimated 2,000 people suffer from the illness worldwide.

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