A strain is a variation of the original form. A viral strain is created when the genetic material (DNA or RNA) mutates or changes. The alternation of the genetic material may cause a change in the characteristics of the virus. Because the genetic material is copied during replication, the new copies of the mutated (changed) virus will share the same changes in characteristics. These new mutation copies of the virus are called a strain. Strains also develop in bacteria, fungi and protozoa.