Correct Option: D

Swine influenza, also called pig influenza, swine flu, hog flu and pig flu, is an infection caused by any one of several types of swine influenza viruses.Swine flu, also known as the H₁N₁ virus, is a relatively new strain of an influenza virus that causes symptoms similar to the regular flu.It originated in pigs, but is spread primarily from person to person.