If tuberculosis is easily spread airborne, how is everyone not carrying it at the least?

TB is is air born, but you are only contagious when you have active TB.  The billions of people in the world with latent TB aren't contagious.  Your immune system walls off the colonies of bacteria in little granules.  

Also, it is a disease that sometimes your immune system can fight off on it's own, and sometimes not.  Lots of people may get infected, get over it, and never even find out.  

Also, a few decades ago the government launched a very effective (and expensive) effort to control TB. It is usually curable.  

The standard TB test tests for antibodies your body makes o fight TB.  Anyone who has ever had TB will test positive, even if they no longer have it. If you test positive, they'd want to do alung X-ray to see if you actually have TB now.  

If you test positive TALK TO YOUR DOCTOR!  This can usually be cured.