Another new swine flu virus has made the leap to humans, though U.S. officials say it seems almost certain the virus hit a dead end.

The Centers for Disease Control reported Friday that a child from Iowa became infected with a new swine flu virus in September, though the case didn´t come to light until November.

The unnamed boy didn´t need to be hospitalized and recovered fully from the illness.

Testing later showed he´d been infected with a swine influenza virus of the H3N2 subtype, different both from the pandemic H1N1 virus and from the seasonal H3N2 viruses that have been circulating in people for decades.

Human cases of infection with swine influenza viruses happen from time to time. Often, though not always, these infections are seen in people who work on pig farms or in proximity to swine herds.

Last summer officials in Saskatchewan spotted a different swine H1N1 virus in two hog farm workers. As in the case of the boy in Iowa, that virus seemed to stop spreading.

"While these cases are rare, they´re not unheard of," Tom Skinner, a spokesman for the CDC, said from Atlanta, Ga.

"We do detect them from time to time and it´s important for us to investigate, which happened in this case. And luckily there wasn´t any evidence of sustained person-to-person transmission."

It isn´t clear how the boy became infected with the virus. He had no known contact with pigs, an eerie echo of the emergence last spring of the pandemic H1N1 virus. H1N1 was first spotted in two children from California who had had no contact with pigs or with each other.

"It was a child, and yeah, very similar to the circumstances that occurred last spring," Skinner said of the Iowa case.

He said the fact that the investigation turned up no other cases and that some time has since elapsed suggests there isn´t any ongoing spread. "I think if there was other transmission going on associated with this case we would have picked it up and we haven´t."

There are a variety of influenza viruses circulating among pigs. According to the CDC, the four main types currently found are H1N1, H1N2, H3N2 and H3N1 viruses.

Prior to the pandemic, a human case of swine flu in the United States was discovered every year or two. But in the three years or so before the pandemic, 12 cases were found.