If you've been running at high speed and suddenly throw it into low gear, you could do yourself some real damage. "It's called the Let-Down Effect. You're under stress, your body's in high gear, and all of a sudden, it's over. You're done. Unfortunately, once you collapse, your immune system leaves you susceptible to whatever nasty germs may head your way." says Diane Bakos with Prevention Magazine.
For example, if you've ever become ill right after finishing a stressful project or after a major crisis, if you seem to get sick on the weekends or during or after vacations, you probably have the Let-Down Effect. So next time, slow down gradually. Get through the stress, then ease yourself into relaxation by doing things like keeping your mind busy. Do mental games and time yourself or throw some extra physical activity into your routine.
There's a technique called somafocus in which you create sensations in which you create sensations of lightness or floating, warmth, or vibration, and these seem to give the immune system a boost.