In line with the virologists’ adage that the only predictable thing about flu viruses is that they are unpredictable, the dog flu has baffled those following it.

“I don’t think we know what this virus is going to do yet,” said one of its discoverers, Dr. Cynda Crawford of the University of Florida veterinary school.

When Dr. Crawford began studying it in January 2004, it had come to her notice as a mysterious cough and pneumonia that killed a third of the greyhounds at a Florida dog track. By the next year, she had found it in seven states and had shown that it could be passed by dogs who just rubbed noses on the street or shared a water dish, and that humans could carry it on their clothes.

It has proved about as deadly as Dr. Crawford predicted. She estimates that by itself, it kills 5 percent of the dogs that catch it. Add the deaths at shelters that eliminate the virus by killing all their dogs and disinfecting their cages, and the total mortality rate is 8 percent.

But it has not spread nearly as vigorously as she expected. It has now been found in 30 states, but almost exclusively in settings where dogs live closely together: shelters, pet stores, kennels and dog schools.

Some veterinarians have found that the dogs that tend to die from it are the “brachycephalics” — dogs with short snub noses. Just as obesity has proved dangerous to human flu victims because of the weight on their chests, being bred to have a short, bent respiratory tract is dangerous for dogs.

A puppy Pug is held by an animal rights activist during a recent press conference. AP Photo/Gregorio Borgia.