I think I know what causes sinusitis & flu ....
I never had sinusitis until my mid-tweenties. My first attack was bad, high fever, blocked nose, painful face, pressure in my ears, bad cough, pain all over my body. I had to take antibotic shots and within 4 weeks, I was starting to become okay.
Months later, I started to feel symptoms of my second attack. I immediately drank alot of water and even went to get an iv solution in the er. I continued to drink alot of water ... i ate bananas and had planty of juices. My attack was less severe than my previous attack.
For me, it was dehydration. I think some people get dehydrated much faster and therefore need to drink more water. I always drank the recommended 8glasses of water, however this was no enough. I now drink 3-4 litres of water everyday.
Make sure you are well hydrated ...