The correct answer is option 4: Virus. Swine flu, also known as H1N1 influenza, is caused by the H1N1 virus. This virus is specifically found in pigs and can spread from pigs to humans. It is mainly transmitted through airborne droplets when an infected person coughs or sneezes. Swine flu cannot be caused by bacteria, fungi, or tapeworms as they are different types of microorganisms. Bacteria are single-celled organisms that can cause infections, fungi are a group of organisms that include molds and yeasts, and tapeworms are parasites that infect the intestines. Therefore, it is important to remember that swine flu is caused by a specific type of virus, not any of the other microorganisms mentioned in the options.