Eggshells are a porous, which allows air, moisture, and some contaminants to pass through. Spoilage bacteria are the most common culprits of eggs turning rotten. When laid, the hen deposits a protective bloom, but this cuticle is not completely impenetrable, so whether or not you wash the egg, bacteria will eventually traverse that porous shell.

Fowlpox is an old viral poultry disease found worldwide and first described in the 17th century. Several strains of the avian pox virus have been identified and named after the primary bird infected.