Multiple members of a family were eventually tested after one, a veterinary worker involved in culling H5N1 infected chickens, fell ill and seemed to set off a chain of infection within his extended family.
H5N1 wasn't initially suspected as the cause of illness, so the veterinary worker was not tested for it when he was sick. Once the virus was identified as a possible cause of the cluster of cases, only one member of the family -- the third person to get sick -- tested positive.
But more laborious followup testing - performed by looking for antibodies to the virus in the blood of surviving suspect cases -- showed that the veterinary worker was indeed infected with H5N1.
The most likely explanation for the cluster is limited human-to-human spread, the WHO said.
"We believe that limited human to human transmission likely occurred among some of the family members," spokesperson Gregory Hartl said Thursday in an e-mail.
"This is not the first time we have seen such an event and, indeed, we did not see any transmission into the community. The behaviour of the virus has not changed, based on the epidemiological evidence from this event, and we remain at Pandemic Phase 3."
The WHO assesses the influenza pandemic threat according to a six level system. Phase 3 -- which the world has been in since 2003 -- involves no or limited human-to-human spread of a novel flu virus that has the theoretical potential to cause a pandemic.
In the Pakistan cluster, the initial case fell ill around Oct. 29 after having direct contact with infected poultry. The man recovered. But while he was ill he was cared for by two of his brothers, neither of whom had any known contact with infected birds.
The first of those brothers died on Nov. 19. While he is considered a probable case, he was never tested and cannot be added to the case count.
The second brother died on Nov. 28. It was his death that prompted health officials to consider H5N1 as a possible cause of the cluster of illnesses. A specimen taken from him before his death tested positive for the virus late last year.
Authorities then cast a broad net looking for other possible cases, identifying five members of the family, three unrelated poultry cullers and a doctor who cared for the family members as potentially being infected.
Of those, only the first or index case and one additional brother -- who likewise had no known contact with sick poultry - have tested positive.
The event sparked a health investigation in the United States as well. Another brother from the family, who lives in Long Island, N.Y., had travelled to the funerals of the men who died.
On his return to the U.S. he informed health authorities of his exposure to possible H5N1 cases. The U.S. Centers for Disease Control in Atlanta sent a plane to collect specimens from the man for testing, but all tests came back negative.
With these newly confirmed cases, the count of known H5N1 cases since late 2003 rises to 378 in 14 countries. Of those, 238 people have died.
