He sounds exactly like ds1 at the same age, similar worries re danger, poison, contamination and health, no sleep due to anxiety etc. We had real trouble thanks to the 'germs' and health curriculum and again with healthy eating.
I'll never forget the major panic attack he had because he was convinced something dripped out of a tree into his open mouth when we were out walking. I couldn't get him to walk, he was frozen with fear and convinced he was going to die.
We've just had one today actually. Ds asked why eggs smell horrible when you cook them. Clever dh told him it's because they contain sulphur! 
Cue refusal to eat lunch, whilst in a panic that eggs contain a poisonous substance. I then had to explain that there are different types of sulphur and if the type found in eggs was poisonous, most of the mammals on the planet would be dead - us included. (I have no idea if there are different types of sulphur by the way - but it worked. 
)
He is 10 now and has just gone into year 6 and we find the worries and anxieties wax and wane dependant on the amount of stress he's under.
The endless questions about 'have I been poisoned' 'am I going to die because of x, y, z' are exhausting, but it really does help to keep an open dialogue going. We eventually came to the realisation that to some extent we had to practise a bit of tough-love and these days we try to nip new fears in the bud by saying things like 'everyone will have touched a conker and not washed their hands at some point in their life and been absolutely fine, I know I did' cue lots of questions about when I did and what happened afterwards (and a little bit of ... ahem .... poetic license and embellishment on my part). Overall we have found that the best way of dealing with things, because once he grabs hold of a worry it just gets bigger and bigger until it becomes crippling for him both at home and at school.
As troutpout said, we've also been clear with him that teacher's are not always as amazingly clever and perfect as children often think they are and sometimes say silly things and even 'shock horror' make mistakes. Same with peers, we've worked on getting him to understand that other children say things that are untrue to impress or frighten each other because they find the response and attention rewarding and the best responses come from their biggest 'lies'. He used to believe every single word anyone said to him, but as he gets older is learning who is less likely to be trustworthy and who to completely avoid believing - although he does still tend to fall for some amazing tall stories, they are usually harmless and designed to impress eg 'I climbed Mt Everest with my Dad when I was 5' type stuff.
The other trick we have tried to teach him to use is to think of his favourite thing and try to really focus on that to stop the 'bad' thoughts from being able to get a look in. He usually chooses to think about building sandcastles on the beach with ds and his siblings, or thinks through a complicated battle plan for his Pokemon games. It doesn't always work and it's taken quite a while to get him to the point where he can do it, but it might be worth a try.
Please don't feel you are silly for posting this, many of us know how exhausting dealing with this level of anxiety on a daily basis can be.