JEFFERSON CITY - A Missouri health official said that cool summer temperatures are partly responsible for the increase in swine flu cases as students head back to the classroom.

Temperatures across Missouri this summer were some of the coolest on record. In Columbia, this summer temperatures were the 9th coolest on record; Joplin had the 8th coolest summer on record, Rolla the 5th coolest.

As winds pushed down cooler air from Canada, temperatures dropped and the air became less humid.

Viruses use their pointy exterior as a mean to attach to a cell's receptors.

"The dryer air seems to make it more conducive to sticking," said Eddie Hedrick, emerging infections coordinator for the Department of Health. "If it's humid spikes try to attach to the cell, but if that's a slippery slope it has trouble attaching."

As winter approaches and the weather becomes dryer, the virus will find it easier to infiltrate human cells.

Although most diseases have petered out by the time summer's high temperatures and humidity rolls around, swine flu spread so quickly because H1N1 presented a different combination among the 144 possible combinations of H's (hemagglutinin) and N's (neuraminidase).

"Seasonal flu are viruses that have been around for a while, circulating for generations. Your immune system has been exposed to them for some time," said Hedrick.

Very few people have any immunity to swine flu, expect for about 30 percent of the 65 and up age group that have some antibodies to the virus from previous exposure to a similar virus, Hedrick said.

"We know it's no worse than seasonal flu except it's harming some people we don't normally see harmed," said Hedrick.The average age of those infected is 12; the average age of those hospitalized is 20, and the average age of those who have died from swine flu is 37, Hedrick said.

Even so, the fatality rate of swine flu remains at .4 percent. Compare that with the Spanish flu epidemic of 1918 where the fatality rate was 2.5 percent.

The rate of contagion of swine flu is similar to other seasonal flu viruses. Each person infected is likely to infect another one or two people, Hedrick said.

"We're anticipating a two fold increase in the number of flu we see (seasonally)," said Hedrick, adding that countries in the southern hemisphere have ably handled the influx of flu cases.

"They've been able to weather the storm with out overwhelming their resources," said Hedrick.

A vaccine for swine flu isn't available yet, but CDC officials have said one should be ready by mid-October.

The H1N1 vaccine will require two shots to be administered three weeks apart.

"Only two weeks after the second shot will you have immunity," said Lisa Barrios, an H1N1 guidance development leader for the CDC. "People who get it are not going to have much immunity until Thanksgiving."

In the meanwhile, like with the seasonal flu, routine hand washing and cleaning are effective in preventing the virus.