Swine flu viruses typically cause illness in pigs, not humans. Most cases occur when people come into contact with infected pigs or contaminated objects moving between people and pigs.
Pigs can catch human and avian or bird flu. When flu viruses from different species infect pigs, they can mix inside the pig and new, mixed viruses can emerge.
Pigs can pass mutated viruses back to humans, and these can be passed from human to human. Transmission among humans is thought to occur in the same way as with seasonal flu -- by touching something contaminated with flu viruses and then touching one's mouth or nose, and through coughing or sneezing. One of the most effective prevention measures is regular hand washing.