If others are sick in the house (cold, flu, stomach bug, etc.) does that always mean everyone else in the house will get it? or can you prevent that from happening?

Surprisingly, the susceptibility to a virus varies a lot individually. Experience from the recent covid-19 pandemic with a very high transmission rate, not all household members are contracted the virus when one member has the disease. The only way to prevent is avoidance but nothing you can do after the exposure and it's hard to predict who will catch the disease after the exposure.