Video narration
How does a virus actually make you sick? Think of a school photocopier: it makes copies when someone uses it. A virus is a tiny infectious particle, but it can't make copies on its own. It must enter a suitable living cell, called a host cell, and use that cell's machinery. Different viruses use different kinds of cells. That's the key: the cell does the copying. For example, SARS-CoV-2 attaches to a suitable cell, gets inside, and directs it to produce more virus copies. Those copies can infect other cells. SARS-CoV-2 is the virus; COVID-19 is the illness it can cause. An infection may cause no noticeable symptoms, mild illness, or severe illness. A virus can make you sick by entering a cell and using it to make copies. The copies can spread, but infection doesn't always cause illness.