Every situation deserves scientific method, so it's impossible for me to explain how everything works. That's fools pill. That what virus theory is for. So you can always understand why you are sick. They play on our simple minded nature.

So, assuming you can find an example of exact same symptoms ( probably can't if you really think about it because the symptoms between people are usually a little or a lot different.), a cold with a headache and sore throat, but not as much congestion or body ache as your siblings had, or you had watery eyes first, but they went strait to the sore throat. My point is that it's usually a little different.

And that's the thing about scientific method, it makes us look closer, see thing we thought we knew in a total different light.

You can read "what really makes us ill" and you will learn about all the things that cause symptoms, and it can all be traced to toxins or parasites.

The contagious aspect has been studied and you can't lots of context, books and articles that criticize how virologists determine contagions.

And The most likely reason you get the impression that illness is contagious is because humans look for patterns, and we share the same foods and are exposed to the household toxins, detergents, frequencies in our houses we share. We all ate the same pasta and tomato sauce (pasta and tomato sauce are actually toxic to our biology, for example), or we all got sick after swimming in the same pool.

These are just some possibilities. So many toxins out there. Even Veggies are bad too. It's insane how much we are lied to.