Question: How do tummy bugs and colds start? like how does the first person get it?

Christian Wirtz answered on 20 Nov 2013:

Hi Jessica,

most of those bugs are caused by bacteria or viruses. Those are single-cell organisms that have been around as long as there has been life on this planet.

In a similar way as we have to eat to live, so do they. The difference is that we eat meat and vegetables and they eat -infect- individual cells. There are some bacteria and viruses that are not harmful to us as they don’t like any of our cells, similar to the fashion that there are things that we could eat but really don’t want to.
So a bug starts when some of these bacteria get into your body (by breathing them in, having them crawl into an open wound, eating them on food, etc.). Usually our body is very good at keeping them out or killing them quickly with its standard security immune system. But sometimes they are too quick and grow into a colony that is too large for the normal system to handle. That is when your body has to actually concentrate on fighting them and makes you feel sick. That has several effects: An increased body temperature can make you produce antibodies faster. The weakness you get from feeling sick means you’ll rest and remain still so your body can really concentrate on this one task at hand.

The first person to get them is the unfortunate one who wanders into the region where they evolved and gets infected through the ways I outlined above. Similar to animals, bacteria also evolved differently in different locations. For example, until about 50 or so years ago there was no AIDS in humans because no human had been in a region where it could be found. But then someone (probably more than one person) ventured there (some jungle where the same virus gave monkeys a cold) and was bitten by a monkey with the HIV virus. That way the person was then the first infected and it spread from him/her. The same is likely to be the case for most bacteria/virus related diseases.