The US Centers for Disease Control and Prevention said on Tuesday it has confirmed 10 more cases of an extremely rare, polio-like condition, across 24 states.
That brings the total number of cases of acute flaccid myelitis to 72, up from 62 earlier this month, mostly in kids under 18.
However, health officials still do not know what is driving the outbreak, which affects the spinal cord, causing the muscles and reflexes in the body to become weak.
AFM is not new, but cases have been on the rise since 2014.
Though the condition remains very rare - affecting only one in a million people in the United States - CDC director Dr Robert Redfield, who took the job in March this year, says it is the agency's top priority.
Scientists are investigating a number of causes, including viruses, environmental toxins and genetic disorders.
But today Dr Redfield urged Americans not to fear the rare polio-like illness sweeping the US as he believes it is not transmissable from human to human.
He told CBS This Morning that he is certain AFM is caused by a specific virus, rather than inter-personal contact.
'I've recently asked again to put together a task force to really try to look at where we're at, and what else could we do to try to solve this problem,' Dr Redfield said.
'The good news is that it doesn't appear to be transmissible from human to human. We don't see clustering in families.'