The pathogens include enterovirus 71, which is known to cause neurological disease; streptococcus suis, which can cause infections like bacterial meningitis in people who have close contact with pigs or with pork products; and dengue, which is transmitted by mosquitoes.
The inappropriate use of steroids, which can suppress the immune system, worsened the illness in a majority of the patients, the doctors said. The World Health Organization (WHO) is expected to advise health care workers to refrain from using steroids in patients with signs and symptoms of the infection, which include severe fever, encephalitis and breathing difficulties.
While not all the microorganisms were present in each patient, doctors concluded the illness was caused by a combination of them and worsened by steroid use.
The WHO sources did not want to be identified because the results of the health organization's investigation have not yet been made public.
"I'm very confident for the reason of the epidemic," said Dr. Phillipe Buchy, chief of virology at the Institut Pasteur in Cambodia and one of the doctors who cracked the case.