I think that saying is a bit unfairly mailgned. For making fire, there is the rule of three, the triangle, and you need all sides to keep combustion going: -

I think that saying is a bit unfairly mailgned. For making fire, there is the rule of three, the triangle, and you need all sides to keep combustion going: - Heat - Fuel - Oxygen knock one out and it falters. We should describe infections the same way. Needed for infection: - Virus/pathogen - Outer defenses penetration (skin/mucous membranes) - Immune system defeat Seeing it that way, it doesn't have to be so mysterious. Some viruses are dormant or almost omnipresent. The infection may be triggered by other conditions changing, including factors that affect your immune system status or efficiency of your outer protections!