Why do I keep getting sick? Tips

Could be a combo of diet and recovery. Training is taxing on your body and does bring your immune system down. I frequently get sick if I'm training a lot, so I just try to be extra careful about washing my hands a lot, drinking plenty of water, etc. Make sure you're getting all your vitamins!

Ease into the buildup more slowly.  Take more rest days during the early stages of the buildup.  And as noted above, rest/recovery, esp. adequate sleep.

x3.

Think of training as medicine.  Every time you do a training session, you're taking a "dose" of training stress.  Your body can only adapt to the training stress so quickly("absorb the medicine") .  Stress of any type compromises our immune system.  Training stress is no different.  Too much training stress too quickly, and the result is your immune system has been weakened to the point that it can't fight off the cold viruses that are around us all the time.  This point varies from person to person.

If you use a tool to estimate training stress and recovery, like TrainingPeaks Performance Management Chart, you can learn where your personal limits for build rate are, and apply as much stress as possible while reducing those colds that cut into your progress.  For example, using the PMC, it calculates something called Training Stress Balance that is a representation of how fatigued an athlete is.  A negative number means that the athlete is fatigued (gaining fitness), and a positive number means they are fresh (losing fitness).  For most people, too much time with a TSB in the range of -20 will result in illness, but I've

ETA:  As a side note, this is also why many people will get a cold a few days after long course races, especially if they went into the race with the minimum amount of training, then raced hard.