As school bells ring again in the coming days, minds will return to reading, writing, math and -- for many people -- the flu.
Autumn is when rates of influenza pick up, building to a peak between November and March. Even though scientists carefully monitor flu viruses in circulation each year and plan ahead to prepare seasonally specific vaccines, influenza infections can be fatally unpredictable.
For the upcoming season, the good news is that nothing so far has raised red flags that the next great flu pandemic is imminent in the United States.
Scientists are, however, keeping their eyes on a few strains of influenza that are causing outbreaks in other countries. A new type of swine flu is also infecting a growing number of people in some states.
This year's seasonal flu vaccine does not protect against the new virus, known as H3N2v, which appears for now to spread from pigs to people but not easily from humans to humans.
"What we're recommending is that people avoid exposure to swine in places like fairs, especially if they're under 5, over 65, or have other risks, like pregnancy, diabetes or asthma," said Michael Shaw, a flu expert with the Centers for Disease Control and Prevention in Atlanta.
"Going by what we're seeing in the southern hemisphere, there were some localized [flu] outbreaks, but it wasn't an unusual season," he added. "But then, flu always has a way of surprising you."
What most people call "the flu" refers to a set of unpleasant symptoms -- fever, sore throat, cough, stuffy nose and muscle aches -- that are far more severe than the common cold. More than 200,000 people end up in the hospital with flu-related complications each year, according to the CDC. Between about 3,000 and 49,000 die from it.
Even though two cases of flu can seem similar, there are a variety of influenza viruses that infect the respiratory tract and cause the flu.
Each virus has an H group and an N group, which describe the proteins involved in invading and infecting. An H1N1 virus, for example, is what caused the pandemic of 1918. Another strain of H1N1 caused the swine flu scare of 2009. Other H1N1 variants have been less virulent.
Influenza viruses are also categorized as type A or B. Type A strains tend to be more serious.
To make the seasonal vaccine each year, scientists look at what people have been getting sick from in the southern hemisphere, where winter strikes during the mostly flu-free days of summer in the U.S.
Based on what they've seen down under, this year's vaccine contains two type A strains -- an H1N1 and an H3N2 variant -- along with one type B virus.
The vaccine does not cover a second B strain that has been circulating during the current winter in Australia and South America, Shaw said. Nor does it cover H3N2v, which as of last Friday, has already infected 224 Americans this summer.
H3N2v was first detected in pigs in 2009, and it infected a small number of people last winter. Even though numbers are already much higher this year, most cases have been mild, Shaw said. Everyone who has been infected has recovered, and the virus responds well to Tamiflu. Most infections have happened after direct exposure to pigs.
If the new swine virus starts showing signs of being able to spread easily from person to person, scientists might rush to develop a vaccine for it.
For now, people with chronic conditions are most at risk, as are kids born after the late-1990s, when a similar virus circulated, leading many people to build up some resistance to it.
The best way to dodge the flu, experts say, is to get the seasonal vaccine this fall. The same advice applies to the pertussis vaccine, which protects against whooping cough, a bacteria-caused respiratory illness that can be dangerous for young children.
Numbers of whooping-cough cases have spiked so far this year in many states, particularly Washington, which declared an epidemic in April. Minnesota and Wisconsin have also experienced recent outbreaks of whooping cough, according to the CDC.
Adults, teens and preteens need a vaccine booster, called Tdap, at least every 10 years. Besides protecting their own health, the booster helps buffer the rest of the population, including infants.
Equally important for avoiding airborne viruses is to wash your hands, cover your mouth when you cough, stick to your own drinking cup and skip work or school if you feel flu-ish.
"The best we can do, even though it's a simple message, is keep making sure people get the routine seasonal influenza vaccine and be very careful with hand-, cough- and sneeze-hygiene," said Andrew Bonwit, a pediatrician who specializes in pediatric infectious diseases at the Loyola University Health System in Maywood, Ill.
"If you have a cold with sore throat, muscle aches and fever," he added, "maybe that's a good enough reason to stay home."