Flu—short for influenza—is an illness caused by influenza viruses. Flu viruses infect the nose, upper airways, throat, and lungs. Flu spreads easily and can cause serious illness, especially for young children, older people, pregnant women, and people with certain chronic conditions like asthma and diabetes. Millions of children get sick with flu each year and thousands are hospitalized. CDC estimates that since 2010, between 7,000 and 28,000 children younger than 5 years old have been hospitalized for flu each year in the United States. Children with chronic conditions like asthma, diabetes, and disorders of the brain or nervous system, and children younger than 5 years old (and especially children younger than 2 years old) are more likely to end up in the hospital from flu.