In November 2012, six workers at the New Territories North animal management center in Sheung Shui became ill and were admitted to the hospital with respiratory tract infections after being in contact with parrots that were in quarantine. It was found that the workers had Chlamydophila psittaci, which is a virus found in over 60 percent of tropical birds but only occasionally crosses over to humans. Researchers at the University of Hong Kong determine that it was unusual for this virus to have infected so many people and thus studied the sixteen parrots being kept in quarantine. They found that the parrots also had another virus that had not previously been discovered. The virus, Psittacine adenovirus HKU1, carried a higher load of Chlamydia psittaci bacteria which together with the adenovirus weakened the immune system of the birds which allowed for an abnormally high level of the Chlamydophila psittaci virus which was enough to cause this outbreak in humans.
This investigation shed some light on why common animal diseases suddenly become more dangerous to humans. All of the infected humans recovered fully from the virus, whereas of the sixteen quarantined birds, fourteen grew sick and were put to sleep. This new virus alone is not a threat to humans but seems to increase the virulence of other viruses to cross to humans.