The number of cases of hand, foot and mouth disease rose dramatically in the first nine months of this year and more adults are getting sick.
More than 4,200 cases of the disease were reported in the city, overtaking the total of the previous two years combined.
The viral infection mainly affects children, but doctors have recently been seeing more cases in adults.
Academy of Medicine president Dr Donald Li Kwok-tung said the large number of outbreaks might be related to humid weather and a low overall immunity in the population.
"In the past, we hardly ever saw cases in adults, but recently we have been seeing more," said the family medicine specialist. "It may be due to a generally lower resistance against infections in the population, especially when people are tired and stressed."
Laboratory tests by the Department of Health found no unusual genetic characteristics in recent viruses causing the disease, but Li said there might still be slight changes in the viral strains that resulted in people having no immunity to them.
Both children and adults who catch the disease usually have a fever, develop painful ulcers inside the mouth and get a rash with vesicles (bubbles filled with liquid) on the hands and feet.
The disease is mostly self-limiting and sufferers recover within a week. Severe cases may develop complications like viral meningitis or encephalitis which could lead to death. There were five such cases this year, but no fatalities so far.
Li explained that the disease seldom occurred in adults because most acquired immunity at young age.
Medical Association president, paediatrician Tse Hung-hing, also noticed more adult cases recently, and generally agreed with Li's explanations.
The Department of Health said the government did not keep age records of the reported cases. The disease was not a statutorily notifiable disease in Hong Kong and only institutions were encouraged to report outbreaks.
As of Wednesday, 652 outbreaks had been reported from institutions such as schools and homes for the elderly, with a total of 4,246 people contracting the disease this year. There were 2,216 affected people last year and 1,370 in 2011.