Recently, a polio-like virus has appeared in 22 states and has been reported by major media outlets such as CBS and NBC News. Reports starting in early November describe 62 cases of a polio-like illness, primarily in children. Officials are looking into 65 other possible cases. Two children have died.

This virus is less severe than the deadly polio virus but is similar to polio in that it causes weakness and paralysis. The Centers for Disease Control (CDC) have stressed that none of the children who developed symptoms have polio.

According to CBS News, this new disease, called acute flaccid myelitis (AFM), is extremely rare: fewer than a million children contract it each year. As Dr. Nancy Messonnier, director of CDC’s National Center for Immunizations and Respiratory Diseases, said in a press briefing, “This is a truly a mystery disease.”

Polio can be contracted in three different ways: nasal secretions, oral secretions, and contact with people who have the virus. However, the CDC does not yet know the long-term effects of AFM or who may be at higher risk for developing the condition.

The good news is that this disease is not a setback in the global effort to eradicate polio. Polio, short for poliomyelitis, is estimated to have plagued humans since 1400 B.C.E. Before polio vaccines were available in the United States in the early 1950s, outbreaks used to sweep through towns in summer months, sometimes causing more than 15,000 cases of paralysis and thousands of deaths in the United States each year.

Though most people recovered quickly, some suffered temporary or permanent paralysis or even death. U.S. President Franklin Delano Roosevelt contracted polio when he was 39 years old and suffered from its effects during his presidency from 1933 to 1945. Before the inventions in the polio vaccine, polio was rampant affecting millions of people nationwide.  

The first polio vaccine was developed by Jonas Salk (pictured above) and came into use in 1955. Albert Sabin developed the oral polio vaccine, which came into commercial usage in 1961. Polio was a common disease of the 20th century but has been eradicated by now in all but three countries (Pakistan, Afghanistan, and Nigeria). Without the polio vaccine and its periodic updates, many more people—children in particular—would be contracting this potentially fatal disease today.

The CDC says that last years uptick in AFM is “notable because there’s already a spike in cases.” According to Vox news, dozens more kids along with the reported, 250 cases, are occurring and health officials do not know why.

This story was updated from its original version on February 21, 2019.