-- Avoiding contact with animals such as pigs may be the best protection if you are among those likely to suffer severe symptoms if you get the flu -- people with lung disease or diabetes, for instance.

H3N2 flu viruses are common among pigs. H3N2 viruses are a subgroup of influenza A viruses and they are known to adapt in humans, Bresee said.

What makes this new version of the H3N2 flu virus different is that it has picked up a gene from the novel H1N1 flu virus that became a pandemic three years ago. This can happen when a person or an animal is exposed to two different viruses at the same time.

Somewhere along the line, H3N2 and H1N1 viruses were present in a mammal at the same time and the "matrix-gene" (or m-gene) from the H1N1 pandemic virus was picked up by the H3N2 swine flu, thus creating a new or variant version of H3N2.

It is this m-gene that has experts on the lookout, because the presence of the m-gene can make it more easily transmissible to humans.

The majority of the children and adults who got the new strain were attending country or agricultural fairs, which is where they came in contact with pigs (the other pig-to-human transmissions occurred in farmers or veterinarians).

Health officials point out this flu is not a foodborne illness. Instead, it spreads like any other flu -- someone sneezes or coughs, spreading the virus to other mammals (humans included) and onto surfaces.

Dr. Lisa Ferguson, a veterinarian for the National Animal Health Policy Programs at the U.S. Department of Agriculture, said this variant of swine flu was first detected in 2010. Bresee said the first human cases were reported in July 2011.

Most of the people infected have been children; among the 16 cases this year, only three were adults, which is also consistent with what was seen last year, Bresee said.

CDC researchers said that while the genetic makeup of the flu strains found in all three states is similar, they do not believe the cases in Hawaii, Indiana and Ohio are related.