Flu is predictable. It tends to gradually increase in the United States in late fall and early winter, reaching a peak in January and February. In Florida, the peak of flu season coincides with the peak of tourist season often reaching epidemic proportion. This is not surprising, as travelers carry the virus throughout the country. We know when it is likely, and can take precautions to avoid the spread of the virus, minimize it's effect, or effectively treat the disease.
Experts have some additional concerns about influenza.
Each year, the virus modifies itself in it's origins. Scientists tells us the virus usually gets it's start in Asia, and usually involves close proximity of birds, pigs and humans. As a new strain begins to spread, it is identified and vaccines are developed to try to prevent the most common strain or strains each year. The virus must be grown in eggs in sufficient quantity to provide enough vaccine for distribution around the world. Usually, the change in the virus is fairly small, and it's relatively easy to make the needed changes in the vaccine.