A strain is a subset of a bacterial species differing from other bacteria of the same species by some minor but identifiable difference.
A strain can also be defined as a population of organisms that descends from a single organism or pure culture isolate. Strains within a species may differ slightly from one another in many ways.
Many diseases, including HIV/AIDS and hepatitis, have multiple strains.
Strain in mechanics can also refer to the volumetric deformation of a solid material subjected to external forces.