(WSYR-TV) — The United States has just reached five million coronavirus cases and continues to lead the world with infections and deaths.

Health officials believe that the actual number of infections could be 10 times higher due to testing limitations and the fact that as many as 40% of people have no symptoms.

Our question is: “Do scientists have any idea why so many people who get COVID-19 never get sick?”

Scientists don’t know the answer to this question, but when looking at the outbreaks, the numbers are shocking.

A Boston homeless shelter had 147 infected residents, but 88 percent had no symptoms despite sharing a living space.

At a Tyson Foods plant in Arkansas, 95 of those who were infected were asymptomatic.

A Diamond Princess Cruise Ship had 750 positive tests and nearly half of the infected did not show symptoms.

As for why, there are some theories being studied.

The Mayo Clinic is looking into previous vaccines that people had for the flu, hepatitis and pneumonia. That may lowers the risk of getting COVID-19.

Other researchers are looking into face masks and whether they may filter just enough of the virus so that those wearing them had mild cases or no symptoms at all.

And the theory that has generated the most excitement in recent weeks is that some people walking among us might already have partial immunity.

But, Dr. Anthony Fauci cautioned that there are multiple likely reasons — including youth and general health — that determine whether any one person gets a serious case of coronavirus.

This reinforces the need, in his opinion, for continued vigilance in wearing masks and social distancing.