How many organisms of M. tuberculosis must you inhale to get TB?

Three.

Weird, isn't it? TB has been with us since at least the Pharaohs of ancient Egypt, and seems to occupy a particular 'ecological niche'. That is, it specialises in a particular life. Mycobacterium tuberculosis divides much more slowly than most other bacteria (roughly every 20 hours, compared with the 20 minutes of E.coli). So how has it survived? Surely it would be out-competed by other species of bacteria capable of dividing more quickly?

Ah, but M. tuberculosis puts its eggs in a different basket. Rather than concentrating resources on dividing, it puts them into its defences. M. tuberculosis is notoriously difficult for the body to get rid of (amongst other things, it has a massively thick, waxy outer coating). So difficult is it to get rid of the organisms that most people who inhale it never do - if M. tuberculosis doesn't cause TB immediately, it usually lies dormant in the lung, waiting for the host's immunity to drop a little. Then, it spreads by chewing its way through the lung to reach a bronchus. The resultant tendency to cough merely completes the bacillus' goal, for each cough then spews out loads of the organisms, sitting in droplets that are so light that they stay suspended for hours... primed for someone else to walk by and inhale them.

And so, with it being so hard to kill, just three organisms are enough to give you TB - even 40 years later, when your immune system is a bit older.