Answer:
There could be many, every person can react differently to different viruses, especially if they have underlying medical problems. The most common serious complication is probably pneumonia as a secondary infection. It could either be a viral pneumonia or a bacterial pneumonia. See the related question below for information about the symptoms of the H1N1/09 flu and about people who may be at high risk for complications from swine flu.
First answer by D4est. Last edit by D4est. Contributor trust: 1252 [recommend contributor recommended]. Question popularity: 3 [recommend question].