swine flu? does this mean human now a days have weak immune system?

It doesn't mean that we have weak immune system. It's a new strain of virus (H1N1 virus or Swine flu) and our body hasn't been exposed to that yet before the epidemic. Most of us are not immune to seasonal flu (Influenca A virus) that's why most of us are still getting immunized with a vaccine flu shot. There are a lot of virus that our body hasn't build immunity like polio, small pox, HIV, and so on. We even have not build immunity for the common cold virus.

For those who have gotten chicken pox, they have developed immunity to it, but there are only a few type of virus we are immune to.

The Swine Flu, or H1N1 virus, is a virus that not many people are used to catching. This is because typically, it only infects 1 or 2 people per year - these people being farmers, who obviously run the highest risk of catching it due to exposure.

Our immune systems were not exposed to Swine Flu. Some people still catch the flu after getting a flu shot. Viruses mutate so we're never fully immune to anything just because we've already had it.

Furthermore, there are people in the US who have had Swine Flu, are medically cleared, and already back at work.

No!

It was originally a virus in pigs but over time it mutated and now is able to infect humans. After awhile the name "swine" referring to pigs, hogs, ect, started affecting the meat industry. People stopped buying bacon, ham, pork, and hot dogs so now the name has been changed to Influenza A. At first it seemed like a huge deal and it was but the threat level is declining drastically. Of course it's also possible that this is just a lie and a way for the wealthy to make money and become even wealthier. Who knows? Remember wash your hands!

No...it just means the virus has evolved to take advantage of another host