The flu virus has arrived in Maine, and right on schedule.

State health officials confirmed Thursday that an adult from Washington County and another from Piscataquis County have been infected with the miserable bug.

About 10 other Mainers in various parts of the state, including the residents of a Lincoln County nursing home, have initially tested positive for the disease.

None of this should come as a surprise to anyone, of course. We all knew it was just a matter of time.

Every year, the state’s medical experts announce that the influenza virus will indeed make its appearance about this time, and that it probably will stick around until about March, after which it will disappear as quickly as it arrived.

That’s one aspect of the nasty disease we can count on – perhaps one of the very few.

But if you’ve ever wondered why, for instance, the flu is almost exclusively a winter disease here in Maine, you’re in good company.

“The flu is completely unpredictable in most of its details,” said Geoff Beckett, Maine’s assistant state epidemiologist with the Bureau of Health. “It’s a fairly complicated issue and we just don’t understand all of the dynamics.”

What we do understand, Beckett said, is that the influenza virus is actually present somewhere on the Earth all of the time.

In Southern Hemisphere countries, such as Australia, the flu season typically runs from April though September, which is winter there.

Its usual arrival here in Maine around November, Beckett theorized, may have something to do with the onset of the busy holidays and the traveling that goes along with the season.

“It’s not as if a single person happened to carry the virus across the bridge in Kittery,” he said. “There are multiple opportunities for the virus to get here. It has been circulating throughout the United States for a few weeks now.”

While no one can say exactly when the flu will come across the state’s borders, he said, the weeks before and after Thanksgiving are usually pretty good bets for its seasonal debut.

“During Thanksgiving and the Christmas season, there are lots of people who travel outside the region and then return,” he said. “The virus is transmitted person to person, so how it gets here is not a mystery. Somebody visited someone elsewhere and brought it back with them. It’s the chain of transmission that we’re not able to figure out.”

Although the flu also can show up in the summer months, he said, the virus is not transmitted efficiently in the warmer weather, so outbreaks are uncommon. The flu has been known to come in on cruise ships, too, which often bring together people from all parts of the world, some of whom might be harboring the virus acquired during a winter outbreak at home.

However the flu arrives, he said, cold weather appears to provide an ideal breeding atmosphere for its rapid transmission. Droplets of flu virus spread easily in close quarters.

“In the winter, people tend to be crowded together indoors more often,” he said. “People’s mucous membranes are a little drier, so that might be a factor. And maybe the virus survives in colder weather better than in warmer weather. But it’s really all speculative when it comes to the flu. We just don’t have a lot of data.”