Flu season is in full swing and numbers are on the rise.

According to Flu.gov, a federal government website managed by the U.S. Department of Health and Human Services, about 5 to 20 percent of the nation's population will get the flu on average during every flu season.

the flu, formally known as influenza, is not what most people refer to as "the stomach flu."

"While vomiting, diarrhea and being nauseous … can sometimes be related to the flu … these problems are rarely the main symptoms of influenza," according to the department's website. "the flu is a respiratory disease and not a stomach or intestinal disease."

This illness is known to spread quickly and easily from person to person. certain groups of people are more prone to catching the seasonal flu, including people older than 65, children under age 2 and anyone who has chronic medical conditions.

“Influenza activity in DuPage County has been increasing since mid-January,” DuPage County Health Department spokesman David Hass said. “Current activity is different from last year’s 2009-2010 season, when a novel strain of influenza resulted in higher rates in October and November.”

According to Hass, current rates today are being compared to high seasonal flu activity levels observed in 2007-2008 and 2008-2009.

the DuPage County Health Department tracks influenza numbers through weekly reports, documenting numbers of student absences and illness.

the worst week this season has been week four, Jan. 23 to Jan. 29, 2011. the health department has numbers documented through Feb. 5.

Schools, on the other hand, are seeing a mix of results from this year's flu season.

According to a report prepared by the DuPage County Health Department, week four of 2011 showed the overall student absenteeism rate (because of influenza) was 6.5 per 1,000 students and week five showed 8.54 per 1,000 students.

the data is collected from about 150 schools in DuPage County.

"Absenteeism numbers are increasing," said Eileen Bell, a school nurse for Glenn Westlake Middle School of Lombard Elementary District 44. "For the most part, students have been home sick due to fever, nausea, strep and the flu."

Bell said there have been some reported cases of the H1N1 virus, but not nearly as many as last year.

"the fevers are probably the result of strep or another strain of the flu other than H1N1," she said. "the flu is more prevalent during the winter months."

Peg Mannion, Glenbard Township High School District 87’s community relations coordinator, said the district has not seen a large increase in flu cases this school year.

District 87 includes all four Glenbard High Schools, which covers nine towns in DuPage County.