Illness and injury can be the two most frustrating factors about running. You build a good base, get into the rhythm, and then you wake up with the flu. Good eating habits will help, and so too will running. Straight after a workout, your immune system will be weakened, with the number of white blood cells reduced. A couple of hours later however, the number of white blood cells will be higher than the pre-workout number. Of course, a number of factors play apart in this reaction, such as how hard you worked. Over time, exercise will benefit your immune system, but sometimes it can have the opposite effect. Research appears to show that the optimal number of miles to run a week is 30. This may seem a lot for beginners, but remember running and distance will help improve your immune system. Runners who ran 60 miles a week in the study had twice as many illnesses a year as those who ran 30. An intresting study on marathon training showed that 12% of runners who ran the marathon got ill, however, only 2% of those who didn't run the marathon, but trained for it got ill. The marathon racers showed a 30-40% decrease in immune system activity straight after the race, so take care after a long hard workout. I should also add that the idea that running in the rain can give you the flu or cold is a myth. Only a virus (or bacteria) can give you an illness.

I wouldn't advise running when your sick, however some runners believe that it is safe to run when all the symptoms are contained to above the neck - for instance a head cold. If you have a chesty cough or infection, you shouldn't run at all until you have fully recovered. If I have a sore throat, I generally run and get away with it. If you run one day and feel worse the next, stop running until you have recovered otherwise you could be out for even longer. Running whilst ill is down to your own discretion. If you do run whilst ill, take it easy and avoid speedwork.

As mentioned in the last paragraph, if there is no daily improvement in your health, do not run. If you have doubts about whether you should run, don't - relapses can occur. Do not do speed training until you have fully recovered. Unfortunately, illness or injury can mean starting all over again on reduced mileage. If you have longer than a week off I would recommend returning to running on reduced mileage.

Asthma. Some sufferers don't exercise simply because they have asthma, this doesn't have to be the case. 25% of Americans have asthma, and many of them continue to run. The fitter you are, the better the condition will get - you will be able to tolerate higher workloads easier, and your breathing will improve. Some good advice for asthma sufferers is to take caffeine. Asthma occurs when airways are constricted, and caffeine is reported to help dilate the airways. As a sufferer it would also be wise to eliminate triggers, somedays this may mean running on a treadmill or swimming. Deep breathing should help, along with warming up and cooling down.