What inhibits viruses from entering and attacking healthy cells?

Interferons are a group of signaling proteins produced and released by host cells in a reaction to the appearance of several pathogens, such as viruses, bacteria, parasites, and also tumor cells. A virus-infected cell will release interferon causing nearby cells to heighten their anti-viral defenses. Interferons are named for their capacity to interfere with viral replication by protecting cells from virus infections.

Interferons also activate immune cells, such as natural killer cells and macrophages. They increase defenses by increasing the expression of major histocompatibility complex antigens. They are antiviral agents which modulate functions of the immune system.