If you’re like most people, you may find it hard to picture SARS-CoV-2, the invisible virus that causes COVID-19. You probably know the virus often attacks the cells lining our respiratory system. The virus spreads to other people in respiratory droplets and smaller aerosols that are expelled while coughing, sneezing, talking, singing, and even breathing.
Here’s a recent photograph taken during a study that may help people better understand the potentially enormous magnitude of viruses produced by infected cells of the human respiratory system.
Newly produced viruses leave their infected cells and can accumulate in huge numbers within mucus. Outgoing, exhaled air scatters mucus in bits ranging from visible droplets to aerosols, which are tiny, nearly invisible specks that can stay suspended in the air for quite a long time after the infected person has left the area.
What does this image tell us? This photograph was produced using a scanning electron microscope. The specimens are actual human cells obtained from a volunteer’s respiratory system. The cells were grown in laboratory dishes, and then infected with a known quantity of SARS-CoV-2. Later, the same cells were examined with an electron microscope.
Electron microscope images are in grey-scale. Here, the original image was colorized to highlight areas of interest. The human lung cells (purple) are covered in hair-like cilia (blue). Those cilia line the inner surface of the airways and help to clear mucus (yellow-green) containing dust and other debris from the lungs. Emerging from the surface of those infected airway cells are many thousands of coronavirus particles (red). For scale, the bar represents 1 um, or one micrometer, which is one millionth of a meter. Those viruses are each 50 – 70 nanometers in diameter. One nanometer (nm) is one billionth of a meter. A human hair is about 75,000 nm in diameter, and a fingernail grows about 1 nm per second! (By the way, in 1 nanosecond, light travels 11.76 inches, about the length of a typical pipe cleaner)
In this study  about 1 million human cells were infected with about 1,000 viruses; in only 96 hours, these cells produced about 10 million viruses.
This explosive growth of the virus may explain how COVID-19 spreads so easily from the lungs to other parts of the body and to other people. Studies have found transmission occurs more efficiently in crowded, indoor environments.
Let’s hope images like this remind us why it is important to avoid indoor crowds; keep physical distance between people; wear face masks when among people; and wash hands regularly.
 SARS-CoV-2 infection of airway cells. Ehre C. New England Journal Of Medicine. 2020 Sep 3;383(10):969.
Photo Credit: Laboratory of Dr. Camille Ehre, University of North Carolina School of Medicine