Could it be blowing from farm to farm in the dirt? Could determined starlings and pigeons be carrying it into poultry houses on their feet? Is it spreading in feed, or being carried on truck tires?

Federal agriculture officials are looking everywhere they can think of for H5N2 bird flu, which has spread to poultry flocks in 14 states and killed or forced the slaughter of more than 39 million birds.

Highly pathogenic avian influenza has never spread like this before in the United States, and it's flummoxed the U.S. Department of Agriculture, farmers and scientists alike.

"The one question that we cannot answer is how is it getting from the migratory birds into these flocks," said Dr. Jarra Jagne of Cornell University, who's studied avian influenza outbreaks around the world.

"We are waiting for a good, warm summer day."

Despite intense efforts to control it, it's continued to spread and the best hope now is for some hot weather to give everyone a break, says Dr. Jack Shere, associate deputy administrator at USDA's Animal and Plant Health Inspection Service (APHIS).

"We are waiting for a good, warm summer day. That's going to help us," Shere told NBC News.

"When we get above 65 it starts to dry the virus out. (At) 85 the virus is practically dead."

Dr. Carol Cardona of the University of Minnesota isn't counting on that help just yet. "Heating up will definitely kill the virus in the environment but in Minnesota that type of weather won't typically happen until mid-July," she said. "The weather is going to help but it's not a silver bullet."

APHIS has counted 177 different outbreaks since H5N2 first showed up late last year. It's affected turkey farms, chicken farms, backyard flocks and has killed broilers and laying hens. In Iowa, the state that produces the most eggs, more than 25 million birds have died or been slaughtered in more than 60 separate outbreaks. Some states have declared emergencies.

"Our usual control mechanisms for dealing with its spread among commercial poultry — they don't work as well," says Shere.

The U.S. has been on the lookout for avian influenza for years, and especially since H5N1 hopped from poultry to people in 2003 and spread to 16 countries.

"We have watched it spread over in Asia. It was only a matter of time before it came to us on the flyways," Shere said. It showed up last fall, first in the form of H5N8, and a few cases of H5N1 - a different strain from the virus affecting people in Asia and the Middle East.

Shere says officials warned farmers and large producers then to double down on biosecurity.

"We saw it coming," Shere said. "We saw it in Washington, we saw it in Oregon. We said you better up your biosecurity; this thing is coming. And it came."

"We saw it in Washington, we saw it in Oregon. We said you better up your biosecurity; this thing is coming. And it came."

The migrating wild birds that spread the virus are the dabbling ducks, which get infected as they float on water and nibble on food just under the surface. They don't necessarily get sick but can spread the virus in their droppings — which are dispersed even further in water.

That made for a bad combination in Minnesota, which has nearly 12,000 large lakes and countless smaller lakes and ponds.

"If you have got migratory birds and they stop at different locations on open water, it is pretty easy to understand," Shere said. "This virus can live in the water for up to 130 days."

And once one farm was infected, lateral spread took over. That's when the virus gets spread by other means - perhaps other wild birds carrying it on their feet, rodents carrying it on their feet, trucks carrying it on their tires, farm workers or contractors carrying it on their boots.

"So you have a double whammy," Shere said.

H5N2's affected commercial or backyard flocks in Iowa, Minnesota, North Dakota, South Dakota, Nebraska, Indiana, Missouri, Wisconsin, Kansas, Arkansas, Oregon, California, Washington and Idaho.

"Sniffer" tests have found the virus in the soil just yards from uninfected farms, Shere says, so everyone must take care. Tests in Europe have suggested the virus can survive being blown short distances in dust.

"We are just guessing that it's moving that way," Shere said "There are some high winds in Minnesota, especially. It moves on a wet, cold wind."

But people may be the most dangerous and persistent carriers, Jagne said. "People coming in have to be closely scrutinized," she said.

Shere agreed. "Let's be honest. The biosecurity is good, but it may not be complete," he said.

"The biosecurity is good, but it may not be complete."

"Perhaps workers aren't completely educated about how important it is to wash your hands and disinfect your boots," he added. "If you don't know where that feed truck has been before it comes to your farm, you should, because it may just have left an infected premise."

It's expensive to do what it really takes to keep a virus out of poultry production facilities, Shere says.

"I heard of a farm today where they don't even allow folks to drive onto the farm," he said. Workers must park, shower in a facility provided for them, change their clothes and get on a bus. Once on the farm, they shower and change again before entering their assigned buildings.

"That is pretty good biosecurity," Shere said. "We have to pull out all the stops. Even with the best biosecurity, we have to do better."

And while right now the biggest threat is to agriculture and to experts of chickens, the biggest fear is that the virus will change just enough to make it infectious to people. No people have been reported infected by this strain yet, but H5N1 and H7N9 have infected hundreds of people and any new avian influenza has the potential to mutate into a form that could become a pandemic.