All coronaviruses sport spiky projections on their outer surfaces that resemble the points of a crown, or "corona" in Latin, according to the Centers for Disease Control and Prevention (CDC). Seven known coronaviruses can infect humans, including the novel coronavirus identified in 2019.
Beneath a coronavirus's pronged exterior lies a round core shrouded in proteins and a "greasy" membrane, Jan Carette, an associate professor of microbiology and immunology at the Stanford University School of Medicine, told Live Science in an email. The core contains genetic material that the virus can inject into vulnerable cells to infect them. So-called spike proteins extend from within the core to the viral surface and allow the virus to "recognize and latch onto" specific cells in the body, Carette said.
"When the spike engages its receptor [on a host cell], a cascade is triggered, resulting in the merger of the virus with the cell," he added. This merger allows the virus to release its genetic material and hijack the cell's internal machinery. "Once this happens, the virus sheds its coat and turns the cell into a factory that starts churning out new viruses."
Several coronaviruses utilize animals as their primary hosts and have evolved to infect humans, too. Precursors to both SARS and MERS coronaviruses appear in bats. The SARS virus jumped from bats to civets (small, nocturnal mammals) on its way into people, while MERS infected camels before spreading to humans. Evidence suggests that the novel coronavirus also jumped from bats to humans after passing through an intermediate carrier, although scientists have not yet identified the infectious middleman creature.
The four most common human coronaviruses — named 229E, NL63, OC43 and HKU1 — did not jump from animals to humans but rather utilize humans as their natural hosts, according to the CDC. These human-borne coronaviruses "have presumably evolved to maximize spread amongst the population rather than pathogenicity," meaning the viruses may opt to maximize their spread rather than harm their human host, Carette said. This may explain why coronaviruses that are transmitted from animals seem to cause more-severe diseases in humans, but the idea remains speculative, he added.
Coronaviruses can be transmitted between humans through respiratory droplets that infected people expel when they breathe, cough or sneeze. A typical surgical mask cannot block out the viral particles contained in these droplets, but simple measures — such as washing your hands, disinfecting frequently touched surfaces and objects, and avoiding touching your face, eyes and mouth — can greatly lower your risk of infection.
The viruses generally cannot survive for more than a few hours on surfaces outside a human host, but people can pick up a coronavirus from a contaminated surface for a short window of time, Dr. Nancy Messonnier, director of the Center for the National Center for Immunization and Respiratory Diseases, said during a CDC news conference in early 2019. Scientists don't yet know how long the novel coronavirus can survive outside a host.