Why are viral diseases epidemic in nature?

During an epidemic, a large number of people become infected with some kind of disease in a fairly short time.

Viruses are essentially enveloped nucleic acid molecules. They are very small in comparison with cells, and many are easily transmitted by airborne droplets, through unwashed hands, etc., after which they enter the cells. Inside the cell, the viral nucleic acid integrates into the genome and replicates, while the viruses themselves multiply and infect more and more cells. In addition, viruses are quite tenacious.

The sick person can easily infect other people, especially if they get into a public place.