(NewsNation) — One of the most highly infectious diseases is now spreading across the country as the U.S. is seeing an uptick in measles cases in at least 15 states.

Measles cases have been popping up in several locations recently, and the U.S. could be looking at a nearly 300% increase from 2023.

In Florida, the ninth case has been reported outside of Fort Lauderdale, infecting a child under the age of five. As the outbreak grows, Florida’s surgeon general has defied public health recommendations and is allowing parents to continue to send exposed, unvaccinated children to school.

If this continues, the U.S. could see up to a 286% increase this year to date.

Michigan had its first case since 2019 and Pennsylvania had nine cases in January. Indiana also reported its first case since 2019.

There are at least 35 cases across 15 states, according to Centers for Disease Control and Prevention data collected through Feb. 21.

Measles is the number one most infections pathogen for children under the age of five and as vaccination rates in public schools drop, it’s spreading more easily.

Measles is airborne and can linger in the air for up to two hours after an infected person is present. The illness also has an incubation period of seven to 21 days and people are contagious for four days before they develop the characteristic measles rash.

Another reason measles can be contagious among young children is that the first Measles Mumps and Rubella (MMR) vaccine isn’t given until 12 to 15 months of age, leaving babies unprotected.

Symptoms may start out similar to a cold, with a very high fever up to 104, a runny nose and then within three to five days, children will show a blotchy red rash.

The CDC says one to three out of 1,000 could result in death because of other illnesses such as pneumonia and other complications.

The spike can be attributed to an increase in vaccine infections and anti-vaccine sentiment spread by misinformation about vaccines, which are a critical tool for public health. Socioeconomic factors can also play a role, as people may struggle to access regular health care.

Vaccines are critical for public health to establish herd immunity and protect those too young to get vaccinated or who have medical conditions preventing them from getting the vaccine.

The measles vaccine came out in the 1960s and in the past few decades, measles was all but eradicated in the U.S., a huge public health achievement given how infectious it is. Prior to the vaccine, measles was a major cause of death for children globally.

Prior to 1963, the U.S. would have about 400,000 measles cases a year and by the early 2000s, there were less than 100 cases per year.