Which body system's main function is to fight infection and disease

it's the immune system which does that

to increase speed you need to decrease frictiona dn drag hense why sprinters wear tight track suits to keep tight against there skin to keep away from wind and decrease the amount of drag you are carrying.a swimmer will wear the tightest swim suit they can possibly fit into (i was a size 28 swim suit at meets i wore a   size 23) why? because the tighter your clothing is the less drag and negative force you have. the heavier the clothe the slower and more wind gets caught. cotton tends to hold onto alot of sweat and water making it slower then say a track suit (no idea what they are made of but they are really comfy around your answer )