Although there is no absolute proof that the Zika virus is behind the surge in microcephaly in Brazil and outbreaks of Guillain-Barré syndrome in six countries, the world’s leading health authorities are close to certain that it is.

“With each passing day, the evidence that it is the cause mounts,” Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention, said recently.

Dr. Bruce Aylward, who is leading the World Health Organization’s response, said on Friday: “At this time, the virus is considered guilty until proven innocent.”

Many rumors blaming other potential causes have arisen, and the authorities have worked hard to debunk them. Here is a look at the most prominent theories making the rounds on social media, along with responses from scientists.