WHO doctors researching the outbreak, which may be linked to severe acute respiratory syndrome (SARS), said the number of cases rose during the winter high season for pneumonia but was now falling off, a spokesman said.
"The outbreak in Guangdong followed a kind of traditional epidemiological curve, which is like a bump, which you would get in winter anyway with pneumonia," spokesman Chris Powell said.
The southern province of Guangdong, where the outbreak the WHO was studying occurred, said on Wednesday 31 people had died of atypical pneumonia by the end of February and 792 had been infected in the province.
Guangdong said it would give March figures next month, although officials said the number of new cases reported this month had fallen sharply.
"Until they officially give us the figures for March, which they've promised to do, you won't be able to say certainly that the numbers have gone down," Powell said.
"But it will have done. It's not like a line which is shooting up through the ceiling."
On Wednesday, the Beijing city government said three people from out of town had died of atypical pneumonia and five found to have the disease, but insisted no local people had contracted it.