Here's a major risk for coronavirus spread that everyone seems to be overlooking

A public health expert warned that the coronavirus can linger in the air and infect others.

Joseph Allen, director of the Healthy Buildings program at Harvard T.H. Chan School of Public Health, told MSNBC's "Morning Joe" that both the Centers for Disease Control and Prevention as well as the World Health Organization are overlooking airborne transmission and focusing instead on COVID-19's spread through droplets and surfaces.

"This is why you clean and disinfect surfaces, but they've ignored airborne transmission," Allen said.

Allen pointed to a choir practice in Washington state where 52 members of a 61-person choir became ill, and two died.

"If you go through a process of elimination through these modes of transmissions, it's clear airborne happened there," Allen said. "Was it fomite or contaminated surfaces? Unlikely -- each person handled their own chair and did hand washing. Was it large droplet? Maybe that contributed, but if I was a singer I would infect the people right around me. You can't get people 50 people infected unless airborne is playing a role there."

"Here is what tipped me off and caught my eye from doing these forensic investigations," Allen said. "The choir practice happened at 6:30 to 9:00 p.m. at night. You know what happens in buildings at that hour? They turn off the ventilation system. There's clear telltale signs -- with the cruise ship, Biogen and many others -- that airborne [transmission] is happening. I don't understand the reluctant to acknowledge this. We need to acknowledge it if we are going to put in the appropriate controls get the pandemic under control before a vaccine."