Kawasaki disease, an illness of unknown cause that mostly affects children under 4, is spreading and has infected more than 10,000 people a year for the past two years, a survey said Saturday.

A nationwide survey conducted by researchers led by Jichi Medical University professor Yoshikazu Nakamura showed that 11,581 people were diagnosed with the disease in 2007 and 11,756 in 2008, highlighting an upward trend that began in the mid-1990s.

Patients with Kawasaki disease typically develop a high fever that lasts at least five days. Other symptoms include red eyes and skin rashes. In serious cases, coronary artery aneurysms can cause sudden death.

The survey said 215.3 of every 100,000 children under the age of 4 developed the disease in 2007 and 218.6 in 2008.

During the two-year period covered by the survey, six patients died from the disease.

The research team sent questionnaires to 2,102 hospitals with pediatric departments earlier this year and got responses from 1,540, or about 73 percent.