The polio epidemic was devastating in the 20th century, causing paralysis and even death in children, but the disease was eliminated in 1979. Now, scientists are worried it’s back — or at least a disease very much like it.

This week, the U.S. Centers for Disease Control and Prevention are saying that there has been a jump in a polio-liek illness called acute flaccid myelitis, and nearly half of all U.S. states had seen an infection between January 1 and August 31, according to a CDC report.

AFM affects the body’s nervous system much like polio, attacking the spinal cord and leading to paralysis in many cases. Unfortunately, we don’t have a vaccine for AFM.

AFM showed big spikes in 2014, and there had been 120 cases of the illness in 34 states by the end of 2014. In 2015, that number declined to 21, but through August of 2016 we are up to 50 people already who have been infected.

Scientists aren’t sure of the cause of the illness, although it appears to be from a viral infection. It could also be from environmental toxins or genetic disorders.

“Acute flaccid myelitis (AFM) is a rare illness that anyone can get. It affects a person’s nervous system, specifically the spinal cord. AFM can result from a variety of causes, including viral infections,” says a CDC statement. “Beginning in August 2014, CDC received an increase in reports of people across the United States with AFM for which no cause could be found. Since then, CDC has been actively investigating this illness. We continue to receive reports of sporadic cases of AFM. From January 1 to August 31, 2016, a total of 50 people in 24 states across the country were confirmed to have AFM. What we know about the AFM cases reported since August 2014: The patients’ symptoms have been most similar to those caused by certain viruses, including poliovirus, non-polio enteroviruses, adenoviruses, and West Nile virus. See a list of viruses associated with AFM.”