Please don't take IB as your first line of enquiry - it is far more likely to be due to dietary or stress factors. Address them thoroughly first... and then if you get no joy, begin to address the possible disease mechanisms.
However, as you've specifically asked...
IB is characterised by a sudden reduction in overall egg production, wheezing and sneezing and general respiratory symptoms, and then about six weeks of very poor quality eggs (wrinkled, soft shells, odd shapes). The most telling symptom is that eggs from a bird with IB will have very watery whites - they just don't hold together around the yolk properly and will simply spread out when you crack the eggs onto a saucer.
Confusingly though, there are strains of IB which only affect the reproductive tract, so the respiratory symptoms will not be present. For this reason, you really need to have a lab diagnosis for proper confirmation of this disease.
It is the most contagious poultry disease known, and once it is in your flock, the infection rate will be 100%. This helps to distiguish it from other causes of laying problems, as every single hen will have it if it is indeed IB. If some hens are unaffected, IB is astronomically unlikely.
It is a virus, and cannot therefore be 'cured' - you just have to wait for it to run it's course. Birds affected by a bout of IB will thereafter be immune to further exposure, but may never lay well again, as the virus can damage the uterus/shell gland.
IB is unlikely to prove fatal, as mortality with the virus is notoriously low, however it can leave the hen susceptible to other pathogens such as e-coli or mycoplasma infections, so you have to be vigilant for those symptoms whilst the virus is running its course, more so than worryingabout the IB itself.
Please, as I said, address the environmental issues first though - IB, although a common poultry complaint, is most often not to blame. And even if it is, you can't actually do much to alter the course of the disease in any case. Try and find a simple cause first - and calcium-related issues are usually where you will find the culprit.