Viral illness throughout the household. What might be happening?

We are a household of 5 people with 2 little ones. That means we get to experience the joy and pleasures of viruses the little ones tend to bring home. After a few of those I've noticed a pattern, each viral infection within the household tends to last 2+ weeks.

1. Patient zero brings the virus home and has symptoms
2. One of the adults starts developing symptoms shortly after. Everyone else is fine.
3. Each other person goes down like domino's but with few days in between. By the time the 3rd or last person is ill the 1st and 2nd person are recovered and healthy. In some instances there my be 1 person that never develops any symptoms, or has no symptoms but is very tired/weak.

This got me thinking, and for context I know nothing about viruses or medicine in general. What is happening here?

• Do all people get infected but not everyone develops symptoms?
• Do some peoples immune systems successfully fight off the viral infection completely or partially?

I'm ruling out anyone being able to avoid contact to escape viral infection due to the size and social dynamics of the household.