Testing results from patients indicate that two strains of seasonal flu that are causing most illnesses -- a Type A H1N1 and a Type A H3N2. Health officials are closely watching a third strain -- a bird flu known as Type A H5N1 -- that has sickened tens of millions of animals, but is known to have infected only 67 people in the U.S.

The United States is seeing a second peak in flu cases this season. After doctor’s office visits for flu dipped early this year, they jumped past the previous season peak at the end of last month, with a 30% test positivity rate — and there are still months to go.

The Centers for Disease Control and Prevention lists Illinois in the high category for flu cases, and across the Chicago area, emergency department visits, hospital admissions, and ICU admissions for the flu are currently higher than the peak of last season.

The U.S. is experiencing its most intense flu season in 15 years, with 24 million infections, 310,000 hospitalisations, and 13,000 deaths, according to the Centers for Disease Control and Prevention.