SALVADOR, Brazil — The Zika virus, some Brazilians are convinced, is the inadvertent creation of a British biotech company that has been releasing genetically modified mosquitoes to combat dengue fever in Brazil. Others here and elsewhere see it as a plot by global elites to depopulate the earth and install a “one-world government.”

And after a group of Argentine doctors asserted that a larvicide, not the mosquito-borne Zika virus, was to blame for a surge in cases of the birth defect known as microcephaly, Brazil’s southernmost state went so far over the weekend as to ban the use of the larvicide in its drinking water — even though scientists and health officials insist there is no such link.

Like Zika itself, rumors about it have replicated with viral ferocity through social media and word of mouth, frustrating the Brazilian authorities as they grapple with a poorly understood pathogen whose origins and implications are still something of a mystery. With many of the rumors started and spread abroad, Brazil’s Health Ministry has been scrambling to do damage control.

In a statement on Monday, the government noted that microcephaly, which causes brain damage, was also growing in communities that did not use the larvicide, pyriproxyfen, and declared bluntly, “The association between the use of pyriproxyfen and microcephaly has no scientific basis.”