There are 10 more confirmed cases of acute flaccid myelitis, a polio-like illness that mostly affects children, the US Centers for Disease Control and Prevention said Tuesday, bringing the total number of confirmed cases to 90 in 27 states this year.
There are also 162 possible cases under investigation, 23 more than last week.
AFM is a rare illness that affects the nervous system, especially the gray matter in the spinal cord, and can cause muscle weakness and sudden onset of paralysis. Last month, the CDC said that more than 90% of the 414 patients since 2014 have been children under the age of 4, although adults can also develop AFM.
The agency released a report Tuesday that said 99% of children with confirmed AFM had experienced a viral illness with symptoms such as fever and cough about three to 10 days before the onset of paralysis.
The CDC also seems to be getting closer to determining a cause of the disease. According to the new report, "Clinical, laboratory, and epidemiologic evidence to date suggest a viral association."
However, Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, said Tuesday that the agency is "not ruling anything out."
In the past two weeks, the agency has come under criticism from its own medical advisers for not zeroing in on a virus as a cause and instead also considering causes such as genetic disorders and environmental toxins.
Many of the CDC's outside medical consultants have long thought that a virus -- specifically an enterovirus -- was most likely at the root of AFM.
Enteroviruses are common; they cause about 10 million to 15 million infections a year in the United States, according to the CDC. Though they're around all year long, they're most common in the summer and fall, which is also when AFM peaks.
Typically, enteroviruses cause people to have cold-like symptoms such as fever, runny nose and body aches, and recovery is easy.
Some of the patients whose cases are detailed in the new report tested positive for an enterovirus, but others have not, although it's not clear whether the testing was done in time to catch the presence of the virus.
It's unclear why a relatively small number of people develop paralysis after an enterovirus infection. Even within the same family, several siblings can develop the cold-like symptoms, but only one may become paralyzed.
Two weeks ago, four of the CDC's outside medical advisers on AFM expressed frustration to CNN that the agency insisted on casting its net wide, looking at genetic causes and possible environmental causes.
The experts felt that the agency shouldn't waste its time on those causes, which they said were highly unlikely, and instead should focus on a virus as the most likely reason behind AFM.
"I'm breathing a sigh of relief that they've acknowledged this," Dr. Keith Van Haren, an assistant professor of neurology at the Stanford University School of Medicine, said of the CDC's new report. "It's great."
Van Haren said now the agency can move faster to figure out exactly which virus is causing the problem and how best to treat it.
Dr. Kenneth Tyler, a professor and chair of the department of neurology at the University of Colorado School of Medicine and another adviser to the CDC on AFM, agreed. "I'm glad to see they're moving in the right direction," he said.
Both neurologists praised the CDC for becoming more communicative and responsive about AFM with doctors and patients over the past two weeks.
Jeremy Wilcox, a Virginia man whose son was diagnosed with AFM in September, also praised the agency for becoming more open.
Parents of children with AFM say that for years, their emails to the CDC went unanswered. Last week, Wilcox arranged a meeting between Dr. Anne Schuchat, the agency's principal deputy director, and more than a dozen parents of children with AFM. That meeting is taking place on Tuesday.