The arrival of the new year is bound to bring the spread of infectious diseases to Keep an Eye On in 2025—both emerging and re-emerging. Several factors make 2025 particularly susceptible to disease transmission in the U.S., including declining childhood vaccination rates and the possibility of a new leader at the Health and Human Services (HHS) Department, who may further fuel skepticism about vaccines under the next Trump administration.
Over the next four years, U.S. health policy is expected to be under intense scrutiny, especially if Robert F. Kennedy Jr. secures Senate confirmation to head HHS.
Despite denying that he is “anti-vaccine,” Kennedy has long been a proponent of vaccine-related conspiracy theories. In July, he claimed, “There’s no vaccine that is, you know, safe and effective.”
While Kennedy has stated he won’t seek to ban vaccines—an action he likely lacks the authority to take and which would face immediate legal challenges—he could still use his platform to discredit them and instill doubt, potentially reducing vaccine uptake. If confirmed, he would assume leadership of HHS at a time when childhood vaccination rates are in decline, and exemptions from school vaccine requirements are rising. These growing populations of unvaccinated and under-vaccinated children create ideal conditions for the resurgence of infectious diseases.
“The outlook for infectious diseases to Keep an Eye On in 2025 over the next four years appears grim,” warns Dr. Perry N. Halkitis, dean of the Rutgers School of Public Health. He categorizes the risks into three main areas: reemerging diseases, currently circulating infections, and new infectious threats.
“We have diseases that were under control for decades but are now resurfacing—such as measles—because parents are opting out of childhood vaccinations,” Halkitis explains. “We also have infections like COVID-19 and RSV, which continue to spread rapidly, causing severe illness and fatalities when people fail to keep up with vaccinations.”
Additionally, new infectious diseases to Keep an Eye On in 2025 are beginning to emerge, and scientists are only just beginning to understand their impact.
Here are five infectious diseases to Keep an Eye On in 2025:
Bird Flu
In 2024, the Centers for Disease Control and Prevention (CDC) confirmed 70 cases of bird flu in humans, including the first recorded “severe” case in the U.S. This patient, hospitalized in Louisiana, ultimately succumbed to the illness.
“While tragic, a death from H5N1 bird flu in the United States is not unexpected due to the known potential of these viruses to cause severe illness and fatalities,” the CDC noted in a press release.
Although the agency currently assesses the public’s immediate risk from bird flu as “low” and has found no evidence of human-to-human transmission in the U.S., experts caution that it remains a concern, particularly given the Biden administration’s inadequate response to outbreaks in dairy cattle herds.
“At this point, it’s evident that we do not have a firm grasp on the dairy cattle-associated outbreak,” says Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “The number of cases is rising significantly, yet there is no real strategy in place to curb transmission.”
Measles
A drop in vaccination rates contributed to a global surge in measles cases in 2024, and the U.S. was not spared.
As of early December, the CDC reported more than 280 cases—the highest annual total in five years. Measles is highly contagious and can lead to severe complications.
Most cases this year occurred in individuals who had not received the measles, mumps, and rubella (MMR) vaccine or whose vaccination status was unknown. According to a CDC report, over 7% of kindergartners—roughly 280,000 children—lacked documentation of two MMR doses and were “potentially at risk for measles infection” during the 2023-2024 school year.
Kennedy has previously voiced opposition to the MMR vaccine. If the Trump administration amplifies anti-vaccine rhetoric, immunization rates could decline further, accelerating disease spread.
“Measles remains a significant challenge,” says Osterholm. “All childhood vaccinations are a concern, regardless of who serves as Secretary of Health and Human Services.”
Polio
Although President-elect Donald Trump recently expressed strong support for the polio vaccine, stating he is a “big believer” in its importance, concerns remain about vaccine uptake under his administration—particularly given Kennedy’s influence.
In 2022, one of Kennedy’s advisers submitted a petition to revoke approval for the polio vaccine. Kennedy himself has falsely suggested that polio vaccines were responsible for cancer in his generation, claiming, “It killed many, many, many, many more people than polio ever did.”
While Kennedy acknowledges that the polio vaccine effectively prevents polio, he has questioned whether it may have caused more harm than good, citing a lack of data.
Polio remains endemic in Pakistan and Afghanistan, and as long as the virus circulates globally, unvaccinated children in the U.S. remain at risk of outbreaks.
According to Osterholm, younger generations lack firsthand experience with diseases like polio, mumps, and rubella. “To them, these illnesses feel almost theoretical,”.
Mpox
Mpox, a viral disease related to smallpox, has been primarily reported among gay and bisexual men. It spreads through close contact and can cause a painful rash. The disease is endemic in parts of Central and West Africa.
Previously, the mpox vaccine (Jynneos) was freely available from the federal stockpile, but that policy changed in April. Experts worry that the cost may deter those most at risk from getting vaccinated.
Although mpox can affect anyone, men who have sex with men remain at higher risk. In November, the U.S. confirmed its first case of a more aggressive strain in California in a patient who had recently traveled from Eastern Africa.
“We have not been proactive about addressing this,” Halkitis says. “It’s not being discussed enough.”
Disease X
Disease X is a term used to represent an unknown pathogen that could cause a major outbreak or pandemic. The concept encourages public health officials and scientists to prepare for unforeseen infectious threats.
“There are numerous viruses and bacteria that can infect animals and humans,” explains Ana Maria Henao Restrepo of the World Health Organization. “We already have vaccines, diagnostics, and treatments for some, but many remain unidentified. We refer to these unknown threats as ‘Pathogen X.’”
Recently, a mysterious flu-like illness spreading in the Congo was initially labeled Disease X. However, health officials later confirmed it as a severe case of malaria.
Experts stress that preparing for the next pandemic is crucial, as another outbreak is inevitable—it’s just a matter of time.
“We are going to experience another pandemic,” Osterholm warns. “The pandemic clock is ticking. We just don’t know what time it is.”
