The sun emits three types of UV-light;
UVA that makes up the majority of sunlight,
UVB that is often credited with causing sunburn and skin cancer,
UVC that is even more harmful but is largely blocked by the ozone layer.
We humans have not evolved to handle UVC exposure but neither have most of the viruses and bacteria that cause sickness, making UVC-light disinfection an effective tool in the fight against coronavirus.