It’s thought that the virus spreads through close contact, traveling through tiny droplets and secretions when a patient coughs, sneezes or breathes.

Typically, when a virus infects a cell in the human body, the cell’s so-called innate immune system kicks in if foreign genetic material is detected. This is considered the body’s first line of defense against invading pathogens. The second line of defense is known as the adaptive immune system, which first has to detect foreign invaders before producing antibodies and T cells to counteract the infection.

But as people age, both of those systems can break down.

“We don’t truly know why, but as you get older, the functionality of the innate immune system and adaptive immune system wanes,” said Timothy Sheahan, an epidemiologist at the Gillings School of Global Public Health at the University of North Carolina.