About the Flu
Influenza (flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. There are two main types of influenza (flu) viruses: Types A and B. The influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year.
The flu is not the same as a cold.
A cold is usually mild, and the flu is more series and comes on suddenly and can make you feel very tired with fever, body aches. It can also lead to more serious health problems.
Who is at risk?
Older people, young children, and people with certain health conditions are at higher risk of serious flu complications. The flu virus causes millions of illnesses and thousands of deaths in the U.S. annually. The best way to reduce the risk of flu and its potentially serious complications is by getting vaccinated each year.
How Flu Spreads
Flu is a contagious respiratory illness and can cause mild to severe illness. Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze, or talk. You may be able to spread flu to someone else before you know you are sick, as well as when you are sick with symptoms. People are most contagious during the first three days of their illness. Some otherwise healthy adults may be able to infect others beginning one day before symptoms develop and up to five to seven days after becoming sick.
Prevention
The single most important step in preventing flu is to get a flu vaccine each year. The Flu vaccine is safe and effective. Learn more about flu myths and facts.
Find vaccines
- CCDPH free immunization clinics
- Vaccines.gov
- Cook County Health
- Vaccines for Children (VFC) program (free for eligible children). Find out if your child qualifies.
- Health care providers
- Pharmacies (for children 3 years and older)
Symptoms
Flu can cause mild to severe illness, and at times can lead to death. Flu symptoms usually come on suddenly. People who have flu often feel some or all of these signs and symptoms:
- fever* or feeling feverish/chills
- cough
- sore throat
- runny or stuffy nose
- muscle or body aches
- headaches
- fatigue (tiredness)
- some people may have vomiting and diarrhea, though this is more common in children than adults.
*It’s important to note that not everyone with flu will have a fever.
Core Recommendations (2025-2026)
Who should get vaccinated
- Everyone 6 months and older should get the flu shot (unless a doctor says it is not safe).
- It is extra important for:
- Babies and young children
- Adults age 65 and older
- Pregnant people
- People with long-term health problems (like heart disease, lung disease, or diabetes)
- People who live with or care for someone at high risk
Timing of vaccination
The best time to get vaccinated is in September or October. But vaccination should still be done later in the season if needed (i.e. whenever vaccine becomes available and flu is still circulating).
Other preventive steps
- Practice hygiene: wash hands often, cover coughs/sneezes, avoid close contact with sick people.
- Stay home if sick, especially until fever is gone without using fever-reducing medications.
- Health care providers, caregivers, and people around infants who can’t yet be vaccinated should get vaccinated themselves to protect infants.
For Healthcare Providers
Local Flu Data
Cook County Department of Public Health collects and analyzes data on local flu activity year-round. During periods when higher influenza activity is expected (generally October to May), this information is compiled into a weekly surveillance report that is distributed to our partners in the healthcare community, schools, community groups, and the public. View our influenza surveillance reports. View our respiratory dashboard for more detailed surveillance data on flu, COVID-19 and RSV.
Updated September 29, 2025, 3:18 PM