Siena Craigie

Siena Craigie, 11, gets a FluMist nasal vaccine Monday at Comer Children's Hospital at University of Chicago Medicine. (Phil Velasquez, Chicago Tribune / March 12, 2012)

The breezes are balmy, green shoots are poking up through the dirt, people are wearing shorts. What could possibly put a damper on the bliss of spring?

The flu.

Off to its latest start in 29 years, influenza season has arrived in the U.S., including Illinois.

Flu viruses — no more than microscopic particles — are infecting people across the state, triggering fevers, violent shaking, extreme fatigue, painful body aches, virus-spreading coughs and general misery that can last a week or more.

As of March 3, Illinois was one of the hardest-hit states in the country, both in terms of geographic spread of the illness and amount of flu-like illness activity, according to data collected by theU.S. Centers for Disease Control and Prevention.

That said, this flu season is proving to be "pretty mild," said Melaney Arnold, spokeswoman for the Illinois Department of Public Health.

Local doctors echo that assessment. "We are seeing more patients with influenza in the past few weeks, but we haven't seen the volume that we usually see," said general internist Dr. Daniel Dunham, an associate professor of medicine at Northwestern University's Feinberg School of Medicine.

Illinois health care providers reported very little flu activity until the first week of February, when two outbreaks were reported in long-term care facilities. Soon after that, another pair of outbreaks were reported in similar institutions.

By the beginning of March, flu was being reported across the state. Almost four dozen people were admitted to intensive care units with confirmed cases of influenza, Arnold said.

Many more likely have suffered at home, their cases unconfirmed and unreported. Outbreaks have been reported in long-term care facilities in Peoria, Edwardsville, West Chicago, Chicago and Cook County, Arnold said.

By this week last year, the flu season was wrapping up, apparently having peaked in late January and early February, according to state statistics. Experts are unsure why it started late this year, but say the most predictable thing about the influenza virus is its unpredictability.

"We can't predict the timing of peak activity in the United States nor when the season will end; nor can we predict how severe the season will ultimately be," said Dr. Joseph Bresee, chief of the epidemiology and prevention branch of the CDC's influenza division, in a late February conference call.

One possible reason for the late start could be that the flu viruses circulating this year — including the H1N1 strain that garnered so much notoriety in 2009 — are similar to those that sickened people last year, Bresee said. That could mean the overall population has higher levels of immunity to these strains this season, leading to less transmission and less disease, he said.

Also, he said, the vaccine is a good match to most of the strains circulating so far this season, and more people are getting the vaccine, so that also could lead to less disease.

A milder winter for parts of the U.S. might have played a role too. Some research indicates that transmission of the influenza virus may be enhanced in colder, drier weather. And in colder weather people spend more time indoors, perhaps aiding transmission, Bresee said.

Still, he said, any role weather may have played in the current pattern remains unclear.

Besides being later than normal, the activity this year isn't dramatically different than a typical season so far, said Dr. Michael David, flu historian and assistant professor of medicine for University of Chicago Medicine. "If transmission continues at a high rate into April, then I will have something to discuss," wrote David, who is also an infectious diseases specialist.

Whatever the cause, for some Illinois residents, these warm, sunny days will be spent inside, racked with pain and fever and coughs, and not outside on a bike or strolling at the lakefront.

So how to tell whether what's slowed you down is a cold, which is also caused by a virus, and not the flu?