Sometimes you really want something to be true, but that doesn't mean that it is. That's what happened to activist and former Star Trek star George Takei back in February. His Facebook page has nearly 10 million fans, which is likely why, when he posted a link to an article suggesting a connection between the deeply unpopular agrochemical company Monsanto and the birth defects blamed on the Zika virus, it quickly went viral. Now that Zika-carrying mosquitoes are here in the United States, speculation that pesticides may really be the cause of these birth defects has gained a whole new audience. 

It all began with a six-page report published by a group of doctors working against the use of pesticides in impoverished villages in South America. The report posed a simple question: Instead of looking only at Zika as the undisputed cause of Brazil's outbreak of microcephaly--babies born with abnormally small heads and brains--shouldn't health officials examine the possibility that a pesticide could be to blame instead?

They had a specific pesticide in mind, pyriproxyfen, in use in the area of the Zika outbreak precisely because it causes birth defects--in mosquitoes. And because the pyriproxyfen used was sold by a Japanese company that partners with Monsanto (they incorrectly identified it as a subsidiary), it became a narrative tailor-made for social media: The company already hated for suing farmers over keeping seeds is behind the microcephaly outbreak and sowing fear of mosquitoes in order to sell more of its product.

All this had a real effect. In the face of growing suspicion of pyriproxyfen, one Brazilian state, Rio Grande do Sul, suspended its use of the chemical pending further investigation. That might sound like a smart thing to do--stop and do further research instead of pouring this stuff into the water supply. The only problem is that if the bulk of scientific thought is correct that microcephaly is caused by mosquito-borne Zika, ending a program to eradicate mosquitoes could lead to more cases of microcephaly instead of fewer. If babies are born with brain damage because a smart bit of activism played into people's reasonable distrust of pesticides, that would be tragic indeed.

Why am I so certain that pyriproxyfen does not cause microcephaly? 

1. It's been in widespread use for decades.

The specific product used in Brazil has also been used in France, Spain, and more than 30 other countries with no outbreak of microcephaly. And pyriproxyfen is in common use throughout the United States as well, in such things as anti-flea treatments. There's been no measurable uptick in microcephaly in these places.

2. There's quite a lot of evidence against Zika.

For example, researchers have shown how the virus crosses through the placenta, and it has been found in the brains of fetuses with underdeveloped brains. It is true that scientists have yet to show exactly how Zika causes brains to stop developing, but it's the same genus of virus as West Nile, which is known to damage brains.

3. Zika is most harmful where it hasn't been before.

The doctors who authored the report and others who doubt the Zika-microcephaly connection have posed an entirely reasonable question: Why haven't we seen microcephaly spike in other places, such as Colombia, where Zika is widespread? But there's an equally simple answer: Once you've had Zika, you're immune for life. So where Zika has a history, it's likely no longer causing birth defects, at least not in large numbers.

In other situations where a Zika outbreak has hit a new population all at once, for instance in French Polynesia three years ago, there does indeed appear to have been an uptick in microcephaly and also many cases of Guillain-Barré syndrome, a rare form of paralysis that's been linked with Zika's current outbreak as well.

Can pesticides be harmful or even deadly? Of course they can. There are good reasons to mistrust them, and perhaps the companies that sell them. But when it comes to the current increase in microcephaly, there's compelling evidence that a virus and not a chemical is to blame.