if you have low immunity, then every time you get sick, be assured your body is building immunity against the bug in you, so next time you get the bug you will be able to fight it better.
make sure you are taking in plenty vitamins and minerals, take suppliments if you think your diet is lacking. try to steer clear of people with bugs...(if a carer has a cold i do not allow them into my house because i will get a chest infection every time without fail!)
make sure food you eat is fresh and well cooked,
drink the following at least twice a day every day, it really helps kill bugs and prevents colds etc

Ginger and cinnamon infusion
ingredients
1 cinnamon stick broken in half
juice of whole lemon
good four finger pinch of whole cloves
2-3 inches of fresh ginger (peeled and cut in thin sticks)
1tsp runny honey (local is best)

put everything in a mug, pour boiling water over it, stir and leave for at least five minutes to infuse, drink all the liquid bar an inch or two, leave the ingredients in the remaining liquid, in the refrigerator,
when you want another, add a little honey, more lemon juice and boiling water, leave to infuse again then drink the liquid.
you can use the same ingredients all day, but start a fresh batch each morning.

try adding 2 cinnamon sticks, a good pinch of cloves and some root ginger to a 2l bottle of lemonade, recap it, leave to infuse for a week or two, for a cold alternative to the above drink.

all the ingredients have a role in fighting bugs and infections... trust me both drinks help, before i became ill i ran a 36 place pre school for years (all those snotty noses and uncovered coughs and sneezes!) once i found this recipe i never even had a cold!