URTI = Upper Respiratory Tract Infection ie colds etc.
Although moderate exercise is good for your immune system there is an increasing understanding of the fact that a high training load can temporarily suppress the immune system. This is seen most prominently in marathon runners and ultra-distance runners.
This link has a simple discussion of some of the mechanisms involved and some of the studies that have demonstrated it. I was thinking of this because I was reading about the origin of distance running and came across a mention of Tuberculosis. Before motor cars the rich and wealthy would travel by coach and would employ 'footmen' who would run ahead to clear obstacles or summon help if the coach broke down or got stuck. As roads got better the demands on the footmen got greater and greater who as a result 'seldom survived three or four years, generally dying of consumption.' We can only assume this may have been a combination of crowded sleeping conditions on the road, mixing with a large variety of poor, and possibly infected people, and possibly a suppressed immune system from all that running.
As roads got better and footmen became unnecessary the rich found another use for them - race them and bet on the result! You can read an interesting article about the Basque running footmen.