Officials at the National Institute of Infectious Diseases warned Monday that this summer could see the worst outbreak in a decade of a virus-induced fever that often strikes young children who swim in public pools.

Since the start of the year, the average number of patients per clinic with pharyngoconjunctival fever, commonly called "pool fever," has been higher than in 2004 -- the biggest outbreak on record -- every week except Jan. 9 to 15, according to the institute.

The institute said it has received reports of around 18,400 patients so far through the latest reporting period from around 3,000 designated pediatric clinics. The illness has been reported in all 47 prefectures.

The institute fears the outbreak may be 10 times greater than in 2004.

Reports through early April showed nearly 80 percent of pool fever patients were aged 5 or younger; 90 percent were 9 or younger.

"An outbreak of the biggest scale is feared," an official said, advising people to wash their hands, to gargle and not to share towels.

The main symptoms of pool fever include a fever of around 39, headache, fatigue and sore eyes and throat. The illness usually clears up in three to five days, but in some cases may cause severe complications, including respiratory problems.

Pool fever is caused by the virulent adenovirus and is known to spread through the air by coughing and sneezing, as well as through physical contact, including sharing towels and touching doorknobs.