In Health Watch: H1N1 is certainly making the rounds this fall, but some major cities are catching a break.Karen Brown looks at why that might be the case.Ron Williams is pounding the pavement...to avoid H1N1.Ron says, "The train is the place you really can catch it. The only time I use the subway is when I need to.So Williams walks instead of taking public transportation.But now health officials say...new york and other major cities hit hard by the H1N1 in the spring...are seeing less of the virus this fall.NYC Health Commissioner Dr. Thomas Farley says, '"We're not seeing an increase in the number of people who are coming to the emergency rooms. So that suggests it's not spreading widely."And the same is true in cities like Boston and Philadelphia.Doctors believe that's because so many have already built up an immunity.Dr. Farley says, "After someone's exposed and had the illness they have very high levels of antibodies that will protect them from future infection."Here in New York...health officials estimate that in the spring, up to 1 million people were infected with H1N1. That means that about 10 percent of the population has built up a resistance to the virus.Dr. Martin Blaser says, "If there are a lot of immune individuals in a population then a new agent isn't going to spread very much.'In fact, states in the southeast...like Georgia ... Which were spared the first wave are now seeing a big jump this fall.But doctors warn...the peak of flu season is from december to march...and no matter where you live..you should be vaccinated.Dr. Farley says "Influenza is the most unpredictable of viruses so you're never quite sure where it's going to travel and how quickly it's going to spread."And this new yorker....Ron Williams says, "I wash my hands, I try to avoid people who have it..'...Isn't taking any chances.