FRED DE SAM LAZARO: On the street outside Phnom Penh's most prominent Buddhist temple, the merit bird business is brisk. It's an age-old ritual in many parts of Southeast Asia based on the belief that freeing a caged bird brings merit to one's soul.
But in recent years, these wild birds have come to symbolize something very different to public health workers: potential carriers of H5N1, the avian flu virus.
PRISCILLA JOYNER, Wildlife Conservation Society: Sellers are very interested in whether or not these birds do have avian influenza. And so they're interested in knowing about the health of the wildlife and how this impacts their health, as well.
FRED DE SAM LAZARO: And their livelihood, too.
PRISCILLA JOYNER: And their livelihood, too, yes.
FRED DE SAM LAZARO: Priscilla Joyner is with the Wildlife Conservation Society. Its staff regularly tests samples of wild birds across East Asia for any signs of flu.
PRISCILLA JOYNER: A big concern here in this area is the very close proximity of people living with domestic animals and interacting with wildlife. And this can either be in the home or this can be in the market or in merit bird training. And this close proximity can be enough pressure to allow a pathogen to jump from one species to the next and then lead to a disease that otherwise may not have occurred.
FRED DE SAM LAZARO: Although so far merit birds have been free of avian flu, Joyner says Cambodia is in many ways an ideal Petri dish for its spread. People are always around wild birds and domestic animals and poultry in the markets and in backyards of this mostly rural country still recovering from decades of war.
H5N1 is common but harmless in ducks. It is lethal in chickens. And it's deadly when it does make the cross-species leap to humans. Two-thirds of the 400 people who've contracted bird flu have died.
Cambodia has seen just eight human cases since 2005. Almost all had very close contact with infected chickens. So far, the virus has not spread from human to human.
Still, Dr. Sirendes Vong of the Pasteur Institute says bird flu remains a concern, especially if it infects someone who already has another form of flu, including swine flu that spread widely in recent months.
DR. SIRENDES VONG, Pasteur Institute, Cambodia: Once humans are infected, if they're infected with seasonal flu, that's a possibility for H5N1 to mix with the seasonal flu and to come up with a new virus that would have the potential to be a pandemic one.
FRED DE SAM LAZARO: To spread like seasonal flu.
DR. SIRENDES VONG: Exactly. How deadly? I don't know. But there's a potential to get a virus that is as deadly as H5N1 and as transmissible as...
FRED DE SAM LAZARO: The bird flu?
DR. SIRENDES VONG: ... as seasonal flu or the current swine flu virus.
FRED DE SAM LAZARO: He says Cambodia doesn't have the resources to check on every case, but with agencies like Pasteur, it is monitoring selected sites across the country for any signs of flu in chickens and responding to major outbreaks.
DR. SIRENDES VONG: If there is something, there's a team from the Ministry of Agriculture, from the veterinary services, that would go to the field and investigate the phenomenon. And the difficulties, again, is to come at the early stage so that you would be able also to test at the early stage of the outbreak.
FRED DE SAM LAZARO: And they are hoping the message gets out on how to lower the risk of an outbreak, separating chickens from ducks, for example, keeping kids away from ponds where ducks swim, and improved hygiene around the backyard. It's a message that's gotten through to small farmers like Khieu Nyim.
KHIEU NYIM, farmer (through translator): I heard the news from the TV and radio. I heard that swine flu makes the pigs sick first and also infects human beings. First, I heard that it spread in Mexico, and then it also spread in America.
I take precautions for myself. I clean the pigs and make sure I wear a mask when I enter the cage.
FRED DE SAM LAZARO: No one's sure whether most farmers are adopting such practices or whether most farmers can afford protective gear. And even though there's fear of the bird flu and swine flu viruses mixing, no one's sure when or if such a super-bug might emerge.
Dr. Michael O'Leary heads the World Health Organization office in Cambodia.
DR. MICHAEL O'LEARY, World Health Organization: I think it's largely a theoretical concern at this point, because we have, you know, many kinds of viruses around us all the time. And so while we have to say that it's possible that these two or other viruses may mix and result in a new virus, that's essentially always the case. We can have such a scenario any time.
FRED DE SAM LAZARO: He says the risk of diseases that jump from one species to another has risen in recent decades with dozens of examples, from Ebola to Lyme disease.
DR. MICHAEL O'LEARY: The destruction of forests or the urbanization of people, that's created new opportunities, I think, for new kinds of interaction between humans and animals. Another is the ease with which people move around the world now.
There have been so many emerging diseases in the last few decades. We've seen dozens of new diseases, HIV being only one, most of which did result from a spread of an organism from the animal world to the human world.
FRED DE SAM LAZARO: O'Leary says it will be important to strike a balance, watching for early signs of outbreaks while avoiding socially disruptive measures, like shutting down the merit bird trade or shutting down markets.
GWEN IFILL: For more on how viruses are transmitted from animals to humans and for Fred's reporter's notebook on Cambodia, visit our global health Web site at newshour.pbs.org.