Hi, I'm not a doctor, for a doctor's response you can post in this forum:
http://www.medhelp.org/forums/Respiratory-Disorders/show/128
PTB has to be active to be contagious, and is primarily spread by coughing water droplets infected with TB that are coughed out. Usually you have to "stew" a few hours in an infected persons cough in a confined area to catch it, it's not spread as easily as influenza is.
PTB is considered active if you have clinical symptoms, and your phlegm culture/pcr and/or tuberculin challenge has showed that you are infected with TB.
Basically if you live in a TB endemic area like the Philippines they tend to treat an abnormal x-ray as PTB until proven otherwise, and undetermined PTB as active until proven otherwise. TB will lay dormant in the lungs encased in a waxy cocoon waiting for your immune system do get run down so it can activate for the rest of your life if untreated. About a third the worlds population is infected with latent TB. That's why it's so important to treat inactive latent TB infection even if you feel great. You probably took a four or five drug combination for two months. They must have determined that your PTB was latent or they would have had you continue taking the two drug combo Isoniazid and Rifampicin for another four months.
You can talk to your doctor about some of these things, hope this helps. Take care.