Illinois health officials released a warning Monday as whooping cough cases in the state rose to their highest levels in two decades.

According to the Illinois Department of Public Health, October alone saw 408 cases of whooping cough, also known as pertussis, marking the "largest monthly total in 20 years." The number is more than double the number of cases reported in October 2023, health officials noted.

Preliminary data showed the state has recorded more than 1,900 confirmed and probable cases of whooping cough so far this year, the highest number since 2012.

Illinois' surge in pertussis is in line with a national surge, which comes after a sharp drop in cases during the COVID pandemic.

“The post-pandemic rise in pertussis cases is a cause for concern and IDPH is monitoring this increase closely throughout the state," IDPH Director Dr. Sameer Vohra said in a statement. "Please speak with your healthcare provider to ensure that you and your children are up to date on your pertussis vaccinations (DTap for children under 7 or Tdap for older children and adults).”

State health officials have sent multiple health alerts to health departments and medical providers across Illinois noting the increase and "urging them to be vigilant about identifying cases and reporting them within 24 hours of diagnosis." Schools were also notified and reminders about vaccination requirements were issued, officials said.

Last week, multiple cases of pertussis were reported at a Chicago high school.

According to a statement from Chicago Public Schools, at least five whooping cough cases have been recently reported at Lane Tech, located on the city’s North Side.

According to the Centers for Disease Control and Prevention, there have been more than 23,000 cases of whooping cough reported in the U.S. this year, nearly five times the total number of cases reported in the same time frame between 2023 and 2024.

The CDC says part of the reason for the uptick is more lax observance of mitigation measures like masking, which became more used during the pandemic but are now slowly fading.

Symptoms of whooping cough are typically mild early on, and include congestion, runny nose, watery eyes, fever and a cough. Once the illness progresses, symptoms worsen, with thick mucus accumulating inside airways and causing uncontrollable coughing, according to the Mayo Clinic.

Coughing attacks can cause vomiting, red face, and extreme fatigue.

Infants younger than 12 months of age who haven’t been vaccinated or those who haven’t received the vaccine are most at-risk for severe complications.

Teens and adults typically recover from whooping cough with no lasting effects, but when complications occur, they typically include side effects from strenuous coughing, including bruised or cracked ribs, abdominal hernias or broken blood vessels in the eyes.

The whooping cough surge comes near the same time health officials also warned of an unusual increase in bacterial infections caused by Mycoplasma pneumoniae.

According to the alert from the CDC, infections from the bacteria have been rising since the spring, peaking in August and remaining high ever since. But while the rise itself is larger than previous years, what makes this spike unique is who the cases are impacting most.

"It's really dramatic," Dr. Sharon Welbel, the director of Hospital Epidemiology and Infection Control for Cook County Health, told NBC Chicago. "A dramatic increase in mycoplasma pneumoniae from 2023 at this time of year to what's happening this year ... but also a really dramatic increase in the 2- to 4-year-olds. So that's new."

The latest trend marks a shift from previous years as cases typically are seen in school-aged children and adolescents, the CDC noted.

Here's what to know:

What is whooping cough?

According to the CDC, whooping cough is defined as "a respiratory illness caused by a type of bacteria called Bordetella pertussis," hence why it is also named pertussis. 

The disease is only found in humans as the bacteria attach to tiny, hair-like extensions that line part of the upper respiratory system and then release toxins, causing airways to swell.

How does whooping cough spread?

Whooping cough can spread easily from person to person through the air, according to the CDC.

"When a person with whooping cough sneezes or coughs, they release small particles with the bacteria in them. Other people can then breathe in the bacteria," the agency states.

Those who contract whooping cough can spread the illness once symptoms start and up until at least two weeks after coughing begins, per the CDC.

Symptoms of whooping cough

Symptoms vary depending on a person's age and vaccination status.

Early symptoms typically take anywhere from five to 10 days after exposure to appear. Some might not appear for three weeks after exposure.

The infection typically starts off like a cold, with the following early symptoms potentially lasting one to two weeks:

• Runny or stuffed-up nose
• Low-grade fever (less than 100.4°F)
• Mild, occasional cough

For babies, however, breathing troubles could arise, regardless of if they cough. Many babies who contract whooping cough require hospitalization.

Later symptoms typically include rapid, violent and uncontrolled coughing fits.

"One to two weeks after the first symptoms start, people may develop paroxysms, known as coughing fits. These coughing fits usually last one to six weeks but can last for up to 10 weeks," the CDC notes. "The cough generally gets worse and becomes more common as the illness continues."

Those who experience coughing fits may also:

• Make a high-pitched "whoop" when they inhale after a coughing fit
• Vomit during or after coughing fits
• Feel very tired after the fit, but usually seem well in-between fits
• Have difficulty sleeping at night
• Struggle to breathe
• Fracture (break) a rib

"Those who get these coughing fits say it's the worst cough of their lives," the CDC stated.

Those who are vaccinated may experience milder symptoms.

Some who experience more severe illness may also experience complications like pneumonia.