Flu cases are slowly rising in Illinois in the lead-up to holiday gatherings this week, so what should you know before you get together with friends and loved ones and how do you know if someone could be contagious?
Experts have been warning for weeks of a mutated variant that could make for a particularly challenging flu season. Meanwhile, cases of norovirus, or the “stomach flu,” are also climbing.
Still, while some Midwest states are seeing a “high” number of influenza cases, according to recent data from the Centers for Disease Control and Prevention, Illinois cases remain “minimal.” The Illinois Department of Public Health reported, however, that the latest metrics are showing a rise in cases.
Data from Dec. 7-13 showed Illinois hospital admissions for flu rose to 1%, up from 0.74% the week earlier. That’s still significantly lower from the high of 6.9% reported in February 2025.
Doctors in the Chicago area have also reported seeing a rise in cases, and with at-home testing available, many cases may go unreported.
“We have seen an uptick in influenza,” Dr. Jonathan Martin, an infectious disease physician with Cook County Health, told NBC Chicago in an interview on Dec. 11. “Influenza season is is definitely underway i expect those cases to increase in the coming weeks and especially since everyone’s going to be inside together during the holiday season that’s when in influenza cases typically spike.”
Meanwhile, states like Michigan are reporting “high” levels of transmission.
Dr. Chase Shutak, medical director of Children’s Minnesota, told NBC News positive flu cases at the facility doubled within the past week.
Wastewater data from across the Midwest have shown “high” increases in norovirus, influenza and COVID-19, according to the WastewaterSCAN program.
The CDC released its latest report on flu Friday, estimating that so far this season, there have been at least 4.6 million illnesses, causing more than 49,000 hospitalizations and 1,900 deaths.
Young children, infants through age 4, have been hit hardest so far.
So what should you know and how can you tell if you or someone you know is contagious?
What is the new flu variant?
The new variant, known as “Subclade K,” is part of the H3N2 parent group, or a type of Flu A.
“This mutation has been associated with an increased rate of flu infections in countries that are already witnessing it and also in parts of the United States,” said Illinois Department of Public Health Chief Medical Officer Dr. Arti Barnes.
The new strain’s impact on other countries like Japan and in the UK has left some local doctors “on guard” as they report it is “spreading fast” and “spreading widely.”
Doctors have warned that the new strain will likely lead to an increase in flu cases this winter.
While testing for the specific variant isn’t as common, Influenza A, specifically H3N2, is the dominant strain spreading in the U.S. right now. While anyone of any age can be affected, H3N2 tends to cause more hospitalizations and deaths in older adults.
What’s the difference between Influenza A and B?
When it comes to flu, there are two types that can cause the seasonal illness many experience in U.S. winters: Influenza A and Influenza B.
According to the CDC, Influenza A viruses are responsible for most human illness and flu seasons, and are “the only influenza viruses known to cause flu pandemics.” There are more than 100 different Influenza A subtype combinations currently known.
Influenza B viruses, however, typically change more slowly than Influenza A and only infect humans.
Are vaccines effective against the new strain?
Doctors said current vaccines are expected to offer protection against serious illness, particularly when it comes to children, but the vaccines were not formulated with this variant in mind.
“Because we didn’t know about [this mutation] before the influenza vaccine was created for this season, that variant is not in our flu vaccine this year. So it does make that vaccine likely to be less effective, but not ineffective,” Welbel said, adding that “the one thing that would make a difference based on what we’re seeing in the UK is if we have a population that’s well vaccinated and also practicing good respiratory hygiene around others to prevent the spread to those who are most vulnerable.”
Barnes stressed that the vaccine was formulated to offer protection against H3N2, which is reassuring, though she noted the new strain is still believed to be more contagious.
“This strain is supposed to be more associated with severe flu season, very rapid spread of the infection. So we think it is very likely more transmissible or more infectious. We don’t know yet if it’s more severe,” she said. “We are looking very closely at countries that are experiencing it like the UK, that have started experiencing it since August, and what we have also discovered through that is that thankfully the vaccine that is in play is still effective towards preventing severe hospitalizations and ER visits from the strain too … it’s about as effective as most flu vaccine strains each season. It is far more effective in children, and children under 8 are a very vulnerable group for complications from flu.”
Flu vaccines are available in the U.S. for anyone 6 months and older.
What are symptoms of flu?
Flu symptoms typically begin within one to four days of exposure, according to the Cleveland Clinic.
Health experts said there are no unique symptoms to watch for when it comes to Subclade K, but they noted cases of this flu strain are likely to be more severe than the common cold.
“Generally, there will be fever, muscle aches, there can be other symptoms such as gastrointestinal symptoms, but people feel pretty sick as opposed to having what we would call the common cold,” Welbel said.
The CDC reports the following symptoms for those with flu:
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.
If you’re unsure whether you have a cold or the flu, here’s a graph to help monitor your symptoms:
Warning signs to watch for
According to the CDC, there are some warning signs people should watch for in children and adults to determine if they need to “obtain medical care right away.”
They include:
In children
Fast breathing or trouble breathing Bluish lips or face Ribs pulling in with each breath Chest pain Severe muscle pain (child refuses to walk) Dehydration (no urine for 8 hours, dry mouth, no tears when crying) Not alert or interacting when awake Seizures Fever above 104 degrees Fahrenheit that is not controlled by fever-reducing medicine In children younger than 12 weeks, any fever Fever or cough that improve but then return or worsen Worsening of chronic medical conditionsIn adults
Difficulty breathing or shortness of breath Persistent pain or pressure in the chest or abdomen Persistent dizziness, confusion, inability to arouse Seizures Not urinating Severe muscle pain Severe weakness or unsteadiness Fever or cough that improve but then return or worsen Worsening of chronic medical conditionsHow long is flu contagious?
Flu can be detected in infected persons at least one day before symptoms begin, and up to seven days after, but those with flu are most contagious around three days into their infection, according to the CDC.
“Some people, including young children and people with weakened immune systems may be contagious for longer periods of time,” the agency reported.
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