well I have had it despite doing my utmost to prevent it, normally it takes about 24 hours before you start feeling sick or having diarrhoea.
Its notoriously difficult to prevent, I washed my hands constantly, if the person is over it they may not infect you, but it is only a maybe!
its not as bad as you think and once you have had it you are immune to catching it again (the strain that is going around now) carry on as you are, keep your distance and be aware of washing your hands before you eat, after the toilet and try not to put your fingers near your mouth (biting nails ect)