I'll give you and example, cruising is one that comes up in the news sometimes. According to you, only the people who come in contact get sick when they have a noro outbreak and not one person came in contact with the virus that didn't get sick. Do you know the odds of that happening?
You would be amazed at how many times you have come in contact with the strep bacteria, flu virus, cold virus, yada yada yada and didn't get sick. Your immune system has a lot to do with when you get sick. Just like the old days of when small pox, polio, etc... Every single doctor who helped those people should have died or been stricken with those/these diseases. Some did but some didn't, why? They had a strong immune system.