It is more popular than you think. Moving between cold and hot zones play a big part in this. In my case, if I move from my house (cold) to the car (hot) trun the A/c on (cold, dah

) and walk to a building (hot) the building is cod again and so on. This would shake the immune system a bit..
On the same note, my A/c at home stopped working a couple of hours ago and the house is getting warmer by the minute. Current tempreature reading is 28C .