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.”

Graham B. White, a medical entomologist who is a consultant to the U.S. Defense Department on disease-carrying insects, called the Argentine doctors’ assertion “ridiculous” and “not credible.”

White said that the larvicide did not work through the nervous system, the part of the developing fetus affected by microcephaly. He noted that it had been approved by the World Health Organization and had been widely used for years to treat drinking water in places where mosquitoes breed.

None of which was mentioned in myriad posts on Facebook and Twitter, where the American actor Mark Ruffalo was among those sounding the alarm, with a post on Monday that was shared nearly 500 times.

Though Brazil is at the center of an epidemic now affecting more than two dozen countries, many of the dubious claims about Zika are born abroad, their purveyors a well-known coterie of critics of genetically modified crops and creatures.

With reputable-sounding names like The Ecologist and Global Research, they produce slick websites that weave facts, half-truths and pseudoscientific analysis into sinister assertions. They trace the hidden hand of “Big Pharma” spreading disease for profit and claim that billionaires like Bill Gates are closeted eugenicists seeking to address overpopulation by promoting poisoned childhood vaccines.

Brazil, where mistrust of government is high, is fertile ground for such theories to gain traction.

Nailma Souza, 40, who owns a nail salon in Salvador, a city that has been hit hard by the outbreak, is certain that Zika was concocted to divert attention away from a cure for cancer that the authorities want to keep secret.

“The Brazilian government wants to patent the drug to make money, but the investigators behind it want to give it away for free,” Souza said after showing a video, purportedly made by a Brazilian actress, pleading for the cancer drug to be released. “The government is involved in this, for sure. They always want to get our money.”

Last week, the Brazilian government used Twitter to counter another popular misconception making the rounds: that vaccines for chickenpox and rubella were to blame for the increase in cases of microcephaly. “Myth,” reads one cartoon the Health Ministry posted, noting that such vaccines are not given to pregnant women. “Making associations between vaccines and microcephaly does a disservice to the public.”

(Another vaccine-related theory: A bad batch of the measles, mumps and rubella vaccine failed to protect against rubella, and the increase in cases of microcephaly was caused by an unreported rubella outbreak, not by Zika.)

Claudio Maierovitch, the Brazilian Health Ministry’s director of surveillance of communicable diseases, said there was a danger that fear and scant scientific information would lead people to ignore the government’s exhortations about protecting themselves from mosquito bites and removing standing water that allows mosquitoes to breed.

“Everything about Zika is new, and there is an aura of mystery about it, which leaves everyone open to new stories and new ideas about where it came from,” Maierovitch said. “The biggest risk is that people don’t do enough preventive procedures because they don’t believe Zika is dangerous or that the disease even exists.”

Michel Misse, a sociologist at the Federal University of Rio de Janeiro, said that rumors and conspiracy theories played an outsize role in Brazilian society, and that the spate of crises buffeting the Brazilian government made people question and doubt authority.

“Given the political moment we are living in,” he said, “a lot of people are not happy with our government, and it definitely plays a role when it comes to believing rumors that blame the government for things that are going wrong.”

In December, the Brazilian government introduced a “Facts and Rumors” website that sought to tackle head-on a dizzying variety of far-fetched stories circulating on the Internet.

Among them are claims that “an army” of Haitian refugees escaping the devastating 2010 earthquake there sought to overthrow Brazil’s government, that the official retirement age was being raised to 95 and that President Dilma Rousseff was overseeing a plan to insert microchips into Brazilian citizens.

“Do not share lies!” warns the website, which asks people to send the authorities the sources of wacky stories they see online. “That way, you’re helping to build a country based on truth and justice.”

Some Zika skeptics have been especially focused on a British company that began releasing genetically modified mosquitoes in Brazil in 2011 in an effort to stem a deadly outbreak of dengue fever. The company, Oxitec, inserts a modified gene into male mosquitoes so that when they mate with females in the wild, they will produce offspring that die before they reach adulthood.

Discover magazine is among the authoritative sources that have disproved the allegation that the engineered mosquitoes are behind the Zika outbreak, born last month on a Reddit board devoted to conspiracy theories. Still, the dubious link has been picked up and expanded by scores of other outlets, including the anti-Western website Russia Today, the British tabloid The Daily Mail and conspiracy-minded websites like Natural News.

“It’s fantasy,” Hadyn Parry, Oxitec’s chief executive, said in an interview. “If children weren’t involved and people weren’t getting sick, it would be quite an amusing fantasy, but this is quite serious and irresponsible.”

Brazilian health officials and scientists say the Oxitec program has reduced mosquito populations in affected areas by up to 90 percent, though its expansion is being held up by the government for technical reasons.

“If politicians start pandering to those opposed to genetically modified mosquitoes without scientific basis to their arguments,” warned Tanjim Hossain, a research fellow at the University of Miami who specializes in mosquito control, “they are going to end up really hurting people.”

Stuck trying to sort out truth from fiction are people like Flora Costa Lima, 34, who was waiting to see a doctor at Salvador’s largest public hospital the other day. Costa Lima, a lawyer, said she was spraying her 18-month-old daughter with repellent every two hours and was rethinking whether to have another child soon.

“I’ve heard that expired vaccines are what’s causing microcephaly in babies,” she said. “I don’t know if that’s true or not, but there is no way I would get pregnant right now. I think it’s best to wait.”

© 2016 The New York Times Company