The phenomenon, unfolding largely on social media, escalated this week when US president Donald Trump retweeted a false video about an anti-malaria drug being a cure for the virus.
It was also revealed that Russian intelligence is spreading disinformation about the crisis though English-language websites.
Experts worry the torrent of bad information is dangerously undermining efforts to slow the virus, especially in the US, where the death toll hit 150,000 on Wednesday – by far the highest in the world.
More than half a million people have died in the rest of the world, according to Johns Hopkins University.
Michael Osterholm, head of the University of Minnesota’s Centre for Infectious Disease Research and Policy, said: “It is a real challenge in terms of trying to get the message to the public about what they can really do to protect themselves and what the facts are behind the problem.”
He said the fear is that “people are putting themselves in harm’s way because they don’t believe the virus is something they have to deal with”.
Rather than fade away in the face of new evidence, the false claims have flourished, fed by mixed messages from officials, transmitted by social media, amplified by leaders like Mr Trump and mutating when confronted with contradictory facts.
In the most recent example, Dr Stella Immanuel promised in a video that promoted hydroxychloroquine: “You don’t need masks. There is a cure. You don’t need people to be locked down.”
However the truth is that US federal regulators last month revoked their authorisation of the drug as an emergency treatment amid growing evidence it does not work, and can have deadly side effects.
Even if it was effective, it would not negate the need for masks and other measures to contain the outbreak.
None of that stopped Mr Trump – who has repeatedly praised the drug – from retweeting the video.
Twitter and Facebook began removing the video on Monday for violating policies on Covid-19 misinformation, but it had already been seen more than 20 million times.
Many of the claims in Dr Immanuel’s video are widely disputed by medical experts.
She has made even more bizarre pronouncements in the past, saying that cysts, fibroids and some other conditions can be caused by having sex with demons, that McDonald’s and Pokemon promote witchcraft, that alien DNA is used in medical treatments, and that half-human “reptilians” work in the US government.
Other baseless theories and hoaxes have alleged that the virus is not real or that it is a bioweapon created by the US or its adversaries.
One hoax from the outbreak’s early months claimed new 5G towers were spreading the virus through microwaves.
Another popular story held that a well-known tech pioneer plans to use Covid-19 vaccines to implant microchips in all seven billion people on the planet.
Then there are the political theories – that doctors, journalists and federal officials are conspiring to lie about the threat of the virus to hurt Mr Trump politically.
Social media has amplified the claims and helped believers find each other.
The flood of misinformation has posed a challenge for Facebook, Twitter and other platforms, which have found themselves accused of censorship for taking down virus misinformation.
Facebook CEO Mark Zuckerberg was questioned about Dr Immanuel’s video during a congressional hearing on Wednesday.
“We did take it down because it violates our policies,” Mr Zuckerberg said.
US representative David Cicilline, a Rhode Island Democrat leading the hearing, responded by noting that 20 million people saw the video before Facebook acted.
“Doesn’t that suggest that your platform is so big, that even with the right policies in place, you can’t contain deadly content?” Mr Cicilline asked Mr Zuckerberg.
It was not the first video containing misinformation about the virus, and experts say it is not likely to be the last.
A professionally made 26-minute video alleging that a US government expert manufactured the virus and shipped it to China was watched more than eight million times before the platforms took action.
The video, titled Plandemic, also warned that masks could make you sick – the false claim Facebook cited when it removed the video down from its site.
Judy Mikovits, the discredited doctor behind Plandemic, had been set to appear on the show America This Week on the Sinclair Broadcast Group. But the company, which operates TV stations in 81 US markets, cancelled the segment, saying it was “not appropriate” to air.
This week, US government officials cited what they said was a clear link between Russian intelligence and websites with stories designed to spread disinformation on the coronavirus in the West. Russian officials have rejected the accusations.
