Fauci says the WHO's comment on asymptomatic spread is wrong. Here's the difference between asymptomatic and pre-symptomatic spread

One oddity is how easily people can get infected by someone without symptoms. But there's a difference between asymptomatic spread and pre-symptomatic spread.

Here's what you need to know:

Asymptomatic spread is the transmission of the virus by people who do not have symptoms and will never get symptoms from their infection. But those infected carriers could still get others very sick.

Pre-symptomatic spread is the transmission of the virus by people who don't look or feel sick, but will eventually get symptoms later. They, too, can infect others without knowing it.

Why is this important to know now?

A World Health Organization official recently said asymptomatic spread "appears to be rare," prompting widespread confusion because doctors and scientists have been saying the opposite for months.

But the WHO's comment "was not correct," said Dr. Anthony Fauci, the top infectious diseases expert in the US.

Evidence shows that 25% to 45% of infected people likely don't have symptoms, Fauci told ABC's "Good Morning America" on Wednesday.

"And we know from epidemiological studies they can transmit to someone who is uninfected even when they're without symptoms," said Fauci, the director of the National Institute of Allergy and Infectious Diseases.

"So to make a statement to say that's a rare event was not correct."

And while the public might use the word "asymptomatic" to describe any infected person who doesn't have symptoms, the bigger concern may be infection from "pre-symptomatic" carriers.

How can I tell if someone is pre-symptomatic or asymptomatic?

You can't. Both types of carriers look and feel normal, though the pre-symptomatic carriers will get symptoms later.

Studies suggest pre-symptomatic spread is more common than asymptomatic spread.

"Detailed contact tracing from Taiwan as well as the first European transmission chain in Germany suggested that true asymptomatics rarely transmit," said Babak Javid, a principal investigator at Tsinghua University School of Medicine in Beijing and an infectious disease consultant at Cambridge University Hospitals.

"However, those (and many other) studies have found that paucisymptomatic transmission (meaning they have extremely mild symptoms) can occur, and in particular, in the German study, they found that transmission often appeared to occur before or on the day symptoms first appeared."

How is it possible to spread coronavirus without symptoms?

"When you speak, sometimes you'll spit a little bit," said Anne Rimoin, an epidemiology professor at UCLA's School of Public Health.

"You'll rub your nose. You'll touch your mouth. You'll rub your eyes. And then you'll touch other surfaces, and then you will be spreading virus if you are infected and shedding" the virus.

How many people get infected by someone without symptoms?

The US Centers for Disease Control and Prevention estimates 40% of coronavirus transmission happens before people feel sick.

In one study, about 4 in 5 people with confirmed coronavirus in China were likely infected by people who didn't know they had it, according to research published in the journal "Science."

"These findings explain the rapid geographic spread of (coronavirus) and indicate containment of this virus will be particularly challenging," researchers wrote.

Many people with coronavirus have no idea they have it -- either because they're asymptomatic, pre-symptomatic, or paucisymptomatic.

The CDC said almost half of the 712 people with coronavirus who were on the Diamond Princess cruise ship didn't have any symptoms when they tested positive.

And a study from Iceland showed 50% of those who tested positive had no symptoms at the time of testing.

How can so many people have or spread coronavirus with no symptoms (yet)?

This coronavirus has a lengthy incubation period -- the time between when someone gets infected to when they start showing symptoms (if they get symptoms at all).

The flu can also be spread without symptoms, but the incubation time is much shorter -- typically one to four days, with symptoms often showing up within two days after infection, the CDC says.

With coronavirus, the incubation period is about three to 14 days, with symptoms typically appearing "within four or five days after exposure," according to Harvard Medical School.

"We know that a person with COVID-19 may be contagious 48 to 72 hours before starting to experience symptoms," Harvard experts wrote.

"Emerging research suggests that people may actually be most likely to spread the virus to others during the 48 hours before they start to experience symptoms."

Are pre-symptomatic carriers more contagious before or after they get symptoms?

"People tend to be the most contagious before they develop symptoms, if they're going to develop symptoms," CNN Chief Medical Correspondent Dr. Sanjay Gupta said.

"They call that the pre-symptomatic period. So people tend to have more virus at that point seemingly in their nose, in their mouth. This is even before they get sick. And they can be shedding that virus into the environment."

If I can't see who's pre-symptomatic or asymptomatic, how do I stay safe?

Wearing face masks and keeping a physical distance from others "can help reduce the risk that someone who is infected but not yet contagious may unknowingly infect others," the Harvard team said.