Bird health is such a huge area to cover. I am going to start with a few things and ad to it as I can.
General health issues
Here is a good site for some things like broodiness, coxi, mareks, bugs, worms, moulting etc.;
I will warn you that they try to sell you their products, but you can decide if that is right for you.
Here is another one with lots of info.; http://www.hobbyfarms.com/livestock-and-pets/chicken-disease-prevention-basics.aspx?cm_mmc=5230086
Here is a good article on preventative healthy measures;
When you have a hen that has a bare back because your rooster is too amorous, then you need a chicken apron. Here is a site with a pattern to make your own; http://backtobasicliving.com/blog/make-a-chicken-saddle/
Or, if you don't have time to make one, here is a site that only charge $5 each with free shipping!
Here is an informative post on chicken pooh! It shows what healthy pooh looks like and some unhealthy pooh too.
Here is some info on vaccines; http://eggcartonlabels.blogspot.com/search/label/vaccinations-The%20Chicken%20Vet
How to vaccinate for Marek's: http://poultryone.com/articles/mareks.html
Worming and Bugs
Here is info on worming your chickens; http://eggcartonlabels.blogspot.com/2012/04/answers-from-chicken-vet-on-worming.html
Here is a site with deworming and debugging info; http://www.swpoultry.com/CindySite/Poultry_health_issues.html