To diagnose swine influenza A infection, a respiratory specimen would generally need to be collected within the first 4 to 5 days of illness (when an infected person is most likely to be shedding virus). However, some persons, especially children, may shed virus for 7 days or longer. Identification as a swine flu influenza A virus requires sending the specimen to CDC for laboratory testing.
Reference: Facts come from the cdc.gov website on virus h1n1flu (swine flu)
the use of face masks and boosting your immune system by taking lots of vitamin-C is still an effective way of preventing the spread of the Swine Flu virus.