Five Infectious Diseases Health Experts Say to Watch in 2026

Infectious diseases that were once considered under control are making a comeback—and a Johns Hopkins expert says 2026 could be a pivotal year for several of them.

Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, points to declining vaccination rates and global travel as key factors driving the resurgence. His list of diseases to monitor starts with measles: as of Tuesday, the U.S. has already confirmed 171 cases this year, following more than 2,200 cases in 2025. Adalja says the country will likely lose its measles elimination status later this month.

Avian influenza remains a top concern. The virus circulates naturally in birds but can jump to other animals—and occasionally humans. Adalja notes the 1918 influenza pandemic, which killed tens of millions worldwide, originated as an avian flu strain.

In Africa, filoviruses like Ebola and Marburg continue spreading into new countries, creating outbreaks that are difficult to contain. Mpox, which causes flu-like symptoms and a distinctive rash, also remains on the watchlist. It spreads through close contact or contaminated materials like bedding, and while a vaccine exists, many high-risk individuals aren't getting it.

Finally, insect-borne diseases are expanding their reach in the U.S. Oropouche virus, transmitted by mosquitoes and biting midges, has emerged as a concern, along with anaplasmosis, a tick-borne illness.

"People know about Lyme disease, but they might not know about anaplasmosis, which can be more severe in the acute phase in terms of fevers, chills, muscle aches and pains," Adalja said.

The through line, according to Adalja: these diseases haven't disappeared—the tools to fight them are simply being underused.