Real-time wastewater surveillance data from 700+ CDC monitoring sites across the US
View Live Flu MapInfluenza A is the most common and most severe form of seasonal flu. Unlike influenza B (which mainly affects humans), influenza A infects both humans and animals, and its ability to mutate rapidly means new strains emerge each year — requiring an updated seasonal vaccine. It is responsible for most flu pandemics in recorded history, including the 1918 Spanish flu, the 2009 H1N1 pandemic, and seasonal outbreaks every winter.
In the US, flu season typically runs from October through March, peaking between December and February. However, wastewater data frequently shows early signals of flu activity in September — weeks before clinical case counts rise.
When someone is infected with influenza A, they shed viral RNA in their waste before symptoms even appear — often 1–2 days before they feel sick. This means a wastewater treatment plant serving a city of 500,000 people can detect rising flu activity days before any of those people visit a doctor or pharmacy.
CDC studies have confirmed that wastewater flu signals precede clinical case reports by 4–7 days on average. For flu season planning — stocking antivirals, adjusting staffing, deciding whether to attend a large gathering — that early warning matters.
On the Outbreak Radar map, orange dots represent influenza A monitoring sites. Dot size reflects the current week's PMMoV-normalized concentration as a percentile of that site's own full history:
A dot showing as Low in December doesn't mean flu is absent — it means levels are lower than this site's historical December readings. Context always matters. Learn more about our methodology on the How It Works page.
Flu activity in the US follows a predictable annual pattern, though its timing and severity vary year to year. Wastewater data typically shows the first signs of rising flu activity in the Southeast and Southwest before it spreads northward. By December or January, most regions are seeing elevated levels. The season usually winds down by April, with most sites returning to near-zero by May.
Using the time slider on the Outbreak Radar map, you can watch previous flu seasons animate — seeing how the wave moved across the country over the winter months.
Common flu symptoms include sudden fever, chills, body aches, fatigue, cough, sore throat, and headache. Most healthy adults recover within 1–2 weeks without medical treatment. However, flu can be serious — and even fatal — for vulnerable groups.
Seek medical care promptly if you experience difficulty breathing, persistent chest pain, confusion, severe vomiting, or symptoms that improve then return with fever. Antiviral medications like oseltamivir (Tamiflu) work best when started within the first 48 hours of symptoms. For detailed treatment guidance, see the CDC flu treatment page.
The following groups are at higher risk for serious flu complications and should be especially attentive to rising flu levels in their area:
Annual flu vaccination is the single most effective prevention measure. The CDC recommends vaccination for everyone 6 months and older, ideally before the end of October. Even in years where vaccine-strain matching is imperfect, vaccination reduces severity and the risk of hospitalization.
Additional prevention measures: frequent handwashing, avoiding close contact with sick individuals, staying home when symptomatic, and improving indoor ventilation during peak season.
Flu wastewater data comes from the CDC National Wastewater Surveillance System, updated weekly. Data is normalized using PMMoV to allow cross-site and cross-time comparison. See How It Works for full methodology details.