Covid threw the infectious diseases playbook out the window this past winter.
Instead of the typical flu season, the U.S. endured a record mix of invasive strep infections, flu, RSV, enteroviruses and other respiratory illnesses that competed with Covid to make most Americans sick at some point.
It might not be over yet.
Recently, the Centers for Disease Control and Prevention reported a spring spike of human metapneumovirus. HMPV is a respiratory virus that's related to RSV (respiratory syncytial virus) and is usually spread through coughing, sneezing or touching surfaces that contain infected respiratory droplets.
"We've seen a ton of HMPV," said Dr. Buddy Creech, a pediatric infectious disease expert at Vanderbilt University Medical Center in Nashville, Tennessee. Most cases are mild, Creech said, involving wheezing and "lots of snot."
Despite the uptick, the CDC said it seems unlikely that HMPV will surge this summer.
"HMPV activity right now is not remarkable," an agency spokesman said in an email, adding that the risk for HMPV spread is low.
If Covid has turned typical seasonal illnesses upside down, what's ahead for summer?
"You can never really predict the future, but I would hope that we'll have a boring summer," said Dr. Anthony K. Leung, an infectious disease specialist at the Cleveland Clinic.
While the U.S. seems to have stabilized for the moment, summer travel is just getting started. Slight upticks in other viruses have put infectious diseases experts on alert as we head into the summer of 2023.
It is impossible to know how many Covid cases are circulating. The CDC isn't tracking cases, although wastewater testing is still underway. Some areas of the country, such New York City, are now seeing high levels of Covid in those samples.
It's too soon to know whether those cases will result in a new wave of severe sickness. As temperatures rise, people will increase their chances of getting infected just by gathering inside in air conditioned areas, behind closed windows and doors.
"Just like in the wintertime, anytime you're indoors together, and someone has it, it's pretty easily spreadable," said Jodie Guest, a professor of epidemiology at Emory's Rollins School of Public Health in Atlanta.
On a hopeful note, hospitalizations from Covid have fallen consistently since the beginning of the year, according to the CDC.
Enterovirus is an umbrella term for many different viruses, such as hand, foot and mouth disease, and even the typical summer cold. These viruses usually spread by coughing and sneezing.
Enteroviruses are often mild, and can cause a range of symptoms, including rashes, fever, loss of appetite and sore throat.
"That's the virus that we anticipate seeing in the summertime," said Dr. Amina Ahmed, chief of pediatric infectious diseases and immunology at Atrium Health Levine Children's Hospital in Charlotte, North Carolina.
In rare cases, enteroviruses can be severe. Enterovirus D68, for example, has been linked to a polio-like condition in children, called acute flaccid myelitis, or AFM. Patients, usually children, develop unusual weakness in their arms or legs.
Before the pandemic, cases would show up in late summer in an every-other-year pattern. Had that pattern continued during Covid, doctors would have expected a rise in 2021. That did not occur.
Now in 2023, it is unclear whether the virus will resume its old patterns. "We're always on the lookout," Ahmed said.
Lyme, norovirus and other vacation bugs
Bugs in different areas of the world can harbor a variety of illnesses.
Three years into the pandemic, an increasing number of Americans have the itch to travel. Doctors say that urge could lead to an increase in travel-related illness, too.
"Always remember that there's some unusual and unpredictable stuff out there," Leung said. "Exercise precaution."
Norovirus, sometimes associated with cruises, can cause days of vomiting and diarrhea.
Many of these illnesses spread similarly, by coming into contact with infectious droplets. The old advice remains: wash hands consistently. If you feel sick, minimize time spent with others to protect them.
But not all summer diseases are spread person to person.
"From spring until late summer and early fall, the infections we worry about are often related to exposure to different insects," said Dr. Michael Angarone, an infectious disease specialist at Northwestern Medicine in Chicago.
Mosquitoes in tropical climates can transmit diseases such as West Nile, dengue and malaria. On May 30, Florida health officials in Sarasota County and Manatee County reported a rare case of malaria in a person who was bitten by an Anopheles mosquito.
Cases of Lyme disease spike in the summer, and there are at least 30,000 new cases of the tick-borne disease in the U.S. each year. In March, the CDC warned about rising cases of a different tick disease, babesiosis, which is becoming more prevalent in the Northeast. Symptoms of babesiosis include:
- Headaches and body aches
- Muscle and joint pain
If you're planning to be out in nature, Angarone said, "make sure that you're protecting yourself from ticks and mosquitoes by using repellents, long pants and long sleeve shirts."
"The quicker you find the tick and get it off, the less likely you are to get an infection," he said.
Mpox is "predicted to spike up a little bit over the summer," said Dr. Michael Saag, associate dean for global health at the University of Alabama at Birmingham.
Mpox is usually transmitted through close physical contact.
"Mpox is not seasonal. It's really about the way people get together,” Guest said. "This is something that we want to be really clear about watching and getting people to vaccinations."
This is critically important for gay and bisexual men who have accounted for the vast majority of cases. Mpox vaccines are available, as well as treatments.