Influenza B virus is a 13- to 14-kilobase single-stranded RNA virus. It has an enveloped capsid and varies between spherical or filamentous morphology. It causes influenza in humans, although not as virulent as influenza A. This may be in part due to the limited species that act as reservoirs for the virus: humans and seals. Influenza B mutates at a slower rate than influenza A. It may be confused with Haemophilus influenzaa type B, a pathogenic bacterium once mistakenly thought to cause influenza.