I think thats correct. Existing ailments can be made worse and thats probably where immunizations should be considered.
My personal opinion is that if your immune system is healthy and strong, you should be able to fight off the flu if you catch it. Our friend got the swine flu when it was going around last year but his wife and 1yr old son did not catch it....they all live together in the same house. He was sick for 5 days then recovered.
So keeping healthy and building up our existing immune systems is always going to be beneficial in helping to prevent the illness. Why his wife and child did not contract could only be because their immune systems were working.

