For about a year now I've been doing the StrongLifts 5x5 weight lifting program. It was about 6 months in that I started to have to pause occasionally while I got over a minor cold - I thought it was just a seasonal thing as winter was setting in.
After a while I'd recover, go back to the gym for a couple of weeks and the situation would repeat. Upon realising the link I'd take a prolonged break from the gym altogether and restart with a big deload (~20%) but within a month or two my immune system would weaken to the point where the process started again.
I'm currently seeking medical advice but after some initial blood tests, and a physical examination there's nothing obvious.
I think I'm being sensible when I exercise, incrementing in 2.5kg stages only after successfully lifted 5 sets of 5 reps, ensuring 48hrs between gym visits and eating extra protein after workouts, extra carbs before, and fruit and vegetables all the time. I'm not even lifting particularly heavy loads - 60kg bench, 110kg dead lift, and 80kg squats are my personal bests.
Can anyone advise on how to strengthen my immune system?
EDIT (to address comments): Sleep - I work out late when the gym is less busy, say 9-10. I get on average 8 or more hours sleep a night, never less than 7. Never had a problem sleeping. Diet - Breakfast is weetabix or other whole grain cereal and whole milk, often a mandarin or similar small orange like fruit. Lunch is usually soup and bread. Evening meal mainly meat, veg and pasta/rice/potatoes. On days where I'm working out I'll eat larger portions than usual. Days after a workout I'll eat additional chicken/tuna throughout the day i.e. as well as normal meals.
Why do I think working out is making me ill? I've gone through the same cycle too many times to realise it's not a coincidence. I wait until I'm well before returning but I'll be run down again after only a few sessions. My previous health regime was football and running and this never happened after physically demanding games.