(CNN)Since the beginning of 2020, when we first started hearing about a new coronavirus, eventually dubbed SARS-CoV-2, our understanding of what it is, how it infects people, who it infects and how we can protect ourselves have all evolved as our knowledge has grown.
But that evolution -- and the changing information and recommendations that accompanied it -- has also sown confusion, and in some cases, deliberate disinformation.
"Just as Covid-19 has spread around the world, so too have rumors, untruths and disinformation. And they can be just as dangerous," Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, said on Wednesday.
Mis- or disinformation has led to people harming themselves based on falsehoods, self-medicating with toxic chemicals or dangerous medications and not taking the precautions that they should be taking, Tedros said. It has also impacted our trust in institutions and health systems, which could result in people turning their backs on new treatments and vaccines if they don't have confidence in them.
Tedros said the WHO and its partners are "calling on all countries to put in place national action plans to promote science-based health information and to combat misinformation. And we call on the media, technology companies, civil society, researchers, and people everywhere to keep the 'infodemic' from spreading," he said.
Here are some of the common myths and misconceptions floating around, and the state of the science as we understand it to date.
Misconception No. 1: Only older people are impacted by the virus
At a rally on Monday, President Trump said, "It affects elderly people. Elderly people with heart problems and other problems, if they have other problems. That's what it really affects. That's it. You know, in some states thousands of people -- nobody young. Below the age of 18, like, nobody."
The fact is people of all ages have been impacted by the virus. While older people are much more likely to get very sick with Covid-19, or die if they're infected, younger people are by no means immune.
In fact, a new study published Wednesday in the CDC's Morbidity and Mortality Weekly Report found that young adults age 20 to 29 accounted for more than 20% of all confirmed Covid-19 cases in the United States during June, July and August -- the highest incidence rate of all age groups.
The National Center for Health Statistics has counted more than 1,800 Covid-19-related deaths in young people under the age of 35, including 419 in people under the age of 25; 851 children under the age of 18 have been hospitalized.
Older people may be more susceptible because they have more pre-existing conditions (called comorbidities) that make a coronavirus infection worse, or their immune systems may be weakened by age.
Some young people also have comorbidities that put them at greater risk of getting very sick. And in some children, the virus can cause their immune system to overreact, creating inflammation and unleashing a cascade of chemical reactions known as a cytokine storm, which wreaks havoc within the body. It's a condition called multi-system inflammatory syndrome in children, known as MIS-C.
Misconception No. 2: Masks don't protect you against coronavirus
This one is perhaps the most contentious and politicized misconception of all. Early in the pandemic, we were told masks weren't important for those of us who were not coming into regular, close contact with sick people. Plus, because of shortages that continue to this day, we were asked to save the N-95 masks for frontline workers.
But masks became a must after we began to understand two very important facts. The first is that people can spread the virus even if they have no symptoms. And the second is that the virus very likely spreads through the air, in small virus-containing droplets called aerosols, and not only by a person coming into contact with an infected surface or large respiratory droplets.
Infectious disease expert Dr. Anthony Fauci addressed the reversal of the guidance on face masks during the CITIZEN by CNN conference on Tuesday. "One of the things that the public needs to understand is that this is an evolving situation," he said.
"We were not aware that 40% to 45% of people were asymptomatic, nor were we aware that a substantial proportion of people who get infected get infected from people who are without symptoms. That makes it overwhelmingly important for everyone to wear a mask," he said, noting that "the data now are very, very clear."
How do masks work? Masks protect other people against the mask-wearer's virus-containing droplets that are expelled into the air via breathing, sneezing, coughing, singing or shouting.
Some studies have found that masks can reduce the amount of droplets that a person breathes into the air by up to 90%. And one study found that masks reduce the transmission of respiratory viruses by as much as 56% percent.
But not all masks are created equal, so choose wisely. Surgical masks -- the paper ones that doctors wear -- have an electrostatically-charged filter that grabs viral particles like a blanket grabs your socks in the dryer. Avoid masks with valves; while they make it a bit easier to breath out, they release unfiltered air -- so, if the wearer is contagious, it doesn't protect others, which defeats the purpose of the mask in the first place.
Misconception No. 3: You can only catch Covid-19 if you've been in close contact with someone who has symptoms
Remember that choir in Washington State? Out of 61 members, there was one symptomatic person and after 2.5 hours of practice on two separate days, 87% of the group became ill. It was early evidence that the virus could spread not just through touch or through respiratory droplets (which tend to fall to the ground quickly and not travel far), but through aerosols, which can linger in the air for hours and travel much farther than 6 feet -- perhaps 20 or more, especially in places with low air circulation.
While the CDC and WHO don't yet explicitly acknowledge this, they are inching in that direction. The CDC went as far as publishing new guidance regarding airborne transmission over the weekend, but the agency reverted to their older guidance on Monday, explaining that the new wording was a draft version that had not yet been fully reviewed.
"The interesting thing about that is it doesn't change anything that we've been saying," said Fauci during Tuesday's conference. "It means wear your mask, it means avoid close contact, it means avoid crowds."
Fauci also reiterated, "Outdoors is better than indoors, because if you have aerosol indoors, you can have some recirculation."
Consider that studies have shown an estimated 80% of cases originate from just about 10% to 20% of people. It's not because these individuals are somehow different, but rather the situations they're in: crowded indoor spaces like bars, restaurants, factories and jails or gatherings that bring people closely together likes weddings or church -- events and spaces that light the match of transmission.
Misconception No. 4: This is like the flu
The President has insisted, "This is the flu, this is like a flu."
But it isn't, really. It's true that both Covid-19 and the flu are caused by respiratory viruses and may share some similar symptoms including fever, fatigue and cough. And in both cases, some people have milder symptoms than others.
But there are also big differences. While the numbers change depending on location and timeframe, according to the CDC's most recent best-guess, the likelihood of dying from Covid-19 -- the infection fatality ratio-- is very low for people under the age of 50. But for people age 50 to 69, it is 0.5%, and for people 70 and older, it jumps up to 5.4%. The overall chances of dying from the flu are about 0.1%.
Additionally, this year there have been more than 200,000 "excess deaths" so far, compared to last year. The biggest difference: Covid-19.
According to the CDC, "These deaths could represent misclassified COVID-19 deaths, or potentially could be indirectly related to the COVID-19 pandemic (e.g., deaths from other causes occurring in the context of health care shortages or overburdened health care systems)."
And, according to statisticians at the CDC, Covid-19 will likely land on the country's top 10 leading causes of death for all ages in 2020. If the current numbers are any indication, it's possible that it will come in at number three, after heart disease and cancer.
Misconception No. 5: Everyone can a get a vaccine this winter
There's been a lot of speculation around when we will have a vaccine, with some optimistic projections as early as October. Several of the developers anticipate having data to share this fall.
But Fauci and other public health leaders have said that it is highly unlikely a vaccine will be available by Election Day. And the US Food and Drug Administration is considering new rules for authorization for a Covid-19 vaccine, according to three sources familiar with the situation, and calculations show these rules would push an authorization beyond Election Day.
That would dash the hopes of President Trump, who has said repeatedly the vaccine could be ready by November 3.
Even if a vaccine were to get emergency use authorization or outright approval this fall, there is physically no way there would be enough doses available immediately for everyone.
"If it's shown efficacious in November or in December, we don't have enough vaccine doses. We'd have a few million in November and maybe 10, 20 million of each in December. That will be enough to ... start vaccinating certain populations but not the whole population," Dr. Moncef Slaoui told me. Slaoui is head of Operation Warp Speed, the government's vaccine initiative.
Slaoui -- who noted that it's not even a given that any vaccine will be effective enough because the data aren't in -- said certain groups, such as health care workers and people who are vulnerable to the disease, would be prioritized. "For the rest of us, it is looking more like mid-2021," he said, a timeline also put forward by Fauci.
Slaoui also circled back to what Fauci and others have been stressing. "Let's stay focused on what science tells to help us overcome this. I'm excited as anyone about getting a vaccine -- but in the meantime there are simple and effective things we can do ourselves: wash our hands, avoid large gatherings and wear a mask."
Nadia Kounang and Andrea Kane contributed to this report.