Our cat Charlotte was treated for pyothorax a few weeks ago. This is an infection in the chest cavity. They had to put a tube in her to drain out all the nasty fluids and try to knock out the infection with antibiotics. We had to have her stay at the vet hospital for 5 days.
We've been medicating her religiously 3 times a day, but it looks like the infection is again getting worse. It's been very frustrating. From what I understand, it can often take multiple types of antibiotics, and can take more than 6 months to knock this thing out.
Here is a really good article by someone who has been through all this (and succeeded in getting rid of it).
If your pet ever gets it, be prepared for a long struggle.