インフルエンザが流行っているけど僕は元気だよ。馬鹿は風邪を引かないって言うからね。

Dear XXX,

How are you doing?
I hope you are working hard and doing well.
How is the flu situation over there?

Here in Japan, many of the schools are closed because of the flu. 
However, I still have not gotten sick so I am still working.
Some people say that the flu does not attack the stupid.

Take care,
[your name]