Why was Swine Flu increasing?

Answer 1

The 2009 H1N1 pandemic Swine Flu was a new subtype of Influenza A that had not been seen before in humans and it was thought that there would be no resistance from being exposed to it before or a virus like it before. Since there was no immunity "built in" and because this virus happened to be a very easily spread virus, it did spread widely and rapidly. Luckily, it was not as severe a flu as anticipated, therefore, the mortality rates were lower than was initially feared.

Now that there is a vaccine (it is included in the 2012-2013 seasonal flu vaccines again), and more people have had this type of flu to build antibodies, it will have less potential to spread as rapidly again, unless it mutates to a new form as influenza viruses readily do, then we may have "cross-immunity" or it may mutate into a new form that our antibodies are not pre-prepared to fight.