Question

why are you more likely to suffer from an infection when you travel by a bus or a train for long hours

Answer 1

Because these all are public transport so it might happen that an ill person may sit on the seat so infection/virus may be left on the seat when the i'll person touchs the seat and when you touch the seat the infection comes on you and you have to suffer

Answer 2

we suffer from infection because there is so much public crowd in the place like bus or train ,some people have some contagious disease which can spray easily and we get infected from that person