Of all the stories about Zika, one of the great mysteries involves its link to the Guillain-Barré Syndrome. Can it involve lightning strikes? Or even past surgeries?

Maybe nothing so outlandish, or could it? Such is the uncertainty surrounding Guillain-Barré (GBS) and Zika.

In the U.S. there has been one case of GBS reported for someone who traveled in an area with Zika, the Centers for Disease Control and Prevention says. Snce 2013, at least 12 countries in Central and South America have reported increases in GBS following the “first introduction” of Zika into those countries.

The Zika virus is transmitted primarily by the Aedes mosquito, and can be spread by sexual contact. The virus infection during pregnancy can cause serious birth defects and is linked with other adverse pregnancy outcomes. On Friday, the Food and Drug Administration Food and Drug Administration recommended “universal testing” of donated blood as another effort to shield the nation from Zika exposure.

So what’s the connection, no manner how tiny, between Zika and GBS?

The CDC is looking closely.

“The CDC is continuing to investigate the link between GBS and Zika to learn more,” says Dr. Jim Sejvar, a neuroepidemiologist at CDC, who has been leading its Zika-related GBS work. “We began conducting enhanced GBS surveillance in Puerto Rico in advance of the island’s Zika outbreak. We are gaining more information about the development and progression of GBS as the Zika outbreak progresses.”

GBS occurs in only a small number of people with recent Zika virus infection, he says. All countries which recently reported increasing numbers of GBS cases have the mosquito species capable of spreading Zika, Sejvar says, noting they have climates conducive to mosquito survival year-round. Generally, cases of GBS occur for no known reason, and true “clusters” of cases of GBS are very unusual.

GBS is an uncommon sickness of the nervous system in which a person’s own immune system damages the nerve cells, causing muscle weakness, and sometimes paralysis. Rates of GBS in the United States and around the world are fairly stable, Sejvar says. That amounts to an estimated 3,000 to 6,000 cases, or 1-2 cases for every 100,000 people, develop GBS each year in the U.S. Most cases of GBS occur for no known reason, and true “clusters” of cases of GBS are very unusual.

Early treatment of GBS with intravenous immune globulin (IVIG) or plasma exchange/plasmapheresis halts the progression of GBS. When treated quickly, patients are less likely to have severe disease or death.

CDC Explores the Connections

There’s a lot of uncertainty involving Zika and GBS.

In 2013, health officials in French Polynesia reported a “concerning increase” in GBS cases that coincided with a large outbreak of Zika, which was the largest outbreak of the virus to that point, Sejvar says. Most people with GBS report an infection before they have GBS symptoms.

Generally, researchers do not fully understand what causes GBS, other than “it’s the body’s response to stimuli,” he says.

“As the (Zika) outbreak continues, we are learning more and we hope to be able to better understand the relationship between the two conditions,” Sejvar says. “We are gaining more information about the development and progression of GBS as the Zika outbreak progresses.”

“Like with other illnesses that may lead to GBS, it’s not known how infection with Zika may trigger GBS,” he adds.

There are some interesting, and sometimes intriguing clues, but they have all to be explored.

For one thing, older people may be at a greater risk of developing GBS, and “that also appears to be the case with Zika,” Sejvar says. “We are working to learn more about what other factors may play a role.”

Oddly, a “number of other non-infectious stimuli such as surgery or a lightning strike have been associated with GBS,” he adds. On rare occasions, vaccinations have been associated with the onset of GBS.

With the transmission of Zika in two small areas of the Miami, Fla. Area, the CDC is working with officials from the state’s Department of Health to identify Zika and any GBS cases, ostensibly to develop or find any links. In addition, the CDC began conducting “enhanced GBS surveillance” in Puerto Rico in advance of the island’s Zika outbreak, according to Sejvar.

He has a warning for doctors:

“Clinicians in the United States should be aware of the potential for GBS cases in travelers returning from areas with Zika, and of the potential for Zika spreading to other areas of the United States,” Sejvar says.