I went through this with DS1 and found it lasted about 4 or 5 months then he never, ever got sick (although he was bf so that would have helped his immunity). So it might be that you are nearly at the end of this bad run.

One thing that can really help with the spread of infections in a centre is to have one of those hand wash solutions at the front desk (you know the ones which you use without needing water?) and ask that all parents, kids and staff use it before entering and leaving the centre. Maybe you could suggest that to your centre.

I hope that things do improve for you. Try not to worry too much about what will happen down the track. I have spent so much time worrying about what will happen if I run out of leave, or work get upset etc but this hasn't happened yet. Just go with it for now. You are lucky that work has been understanding and I am sure they will continue to be that way. Most bosses know what it's like to have babies and children so are happy to make allowances.