In the past 3 years (a little longer), I've had the flu twice. The first time I had it for two weeks, and I continued to attend school while sick (and I sadly didn't infect a single person XD). The second time I had it was over Christmas vacation last year and I was miserable.
My question is this: Does having the flu more than once make me more immune to the disease or more susceptible to it?
The problem with flu is that it keeps undergoing genetic recombination and mutations to make new strains of the virus. If you have had one strain, you are immune to that same exact strain but not any others. So no, having the flu doesn't make you immune to it, nor does it make you more susceptible to it than you were before. It can confer a small bit of immunity if the strain you had before is very similar to a current strain, but that depends on how closely related the strains are.