There are a number of reasons for a weakened immune system.
One of the the most common causes is.........sugar!
Sugar rich foods can cause a fifty- percent drop in the ability of white blood cells to engulf bacteria!!!
1. Reduce Sugar
So to help strengthen the immune system against flu or anything else; reduce your consumption of high sugar content foods, or just eliminate them all together.
2. Probiotics
Many of us suffer from poor digestion and reduced levels of friendly bacteria in the gut. The most common killer of friendly gut bacteria is Antibiotics.
To boost immunity probiotics can be used to boost immune function.
For Your Information - The Russian Military developed a derivative of probiotics to help protect their troops against biological weapons. The probiotic derivative is made up of probiotic cell and DNA fragments and is designed to rapidly and powerfully boost the body's immune system.
Probiotics should help boost the immune system in general not just for Flu.
Oh by the way the probiotics help improve digestion as well amongst other things. Edit