Because B strains of influenza are less genetically diverse than A strains, immunity is often acquired at an early age, officials said.
A vaccinated population also protects those who cannot get the vaccine for medical reasons, she said.
Matt McCall is a freelance reporter for the News-Sun.
Plofsky said about 6 percent of patients at the hospital are presenting these symptoms, where normally 2 percent are this time of year.
Infected people are contagious for five to seven days and should only return to work or school when their fever has been gone for 24 hours, officials say.
While individual cases of flu are not reported in Illinois, 37 people had been sent to intensive care units for flu-related illness in Lake County this flu season, as of late last week, as compared to 48 ICU hospitalizations for the entire 2016-2017 flu season, which begins in late August. One pediatric death has been reported.