Latent infection with TB
TB is actually not regarded as being a highly infectious disease. Under normal circumstances someone with infectious TB (in other words coughing active bacteria) is estimated to infect a further twelve to fourteen people in a year. This may sound a lot but, when compared to a highly infectious epidemic like flu, it is slow - and therefore there should be every reason why it should be controllable if there is suffcient will to do so.
Additionally, most of those who are actually infected never go on to develop active TB at all - instead their own immune systems "wall in" the bacteria in the lungs in a kind of stalemate. This is called "latent infection". Under normal circumstances only one in ten (10%) of those infected go on to develop active TB (normally when the immune system is weakened or off-guard).
That's the good news.
The bad news is how many people are latently infected. The WHO have estimated that 32% of the global population is latently infected (so over two billion people). In the developed world, this percentage will be much, much lower than this, of course.
In Africa the rate of latent infection is estimated to be 80%. That is four out of every five people.
There are around 800 million people living in Sub-Saharan Africa. 80% means that 640,000,000 are infected either latently or actively with TB. This is an astonishing number to try and get your head around because each and every one is under active threat from this disease.
If all these cases were drug-susceptible (as most thankfully are), this is bad enough, but if only a small proportion is drug-resistant, this is almost certainly the largest global health issue facing mankind today.
And unfortunately in concert with HIV/AIDS (or indeed poor diet and living conditions generally) that 10% conversion rate of latent-to-active disease simply goes out the window. To our knowledge, no-one has put a firm figure on it. One estimate suggests that it makes the conversion as much as thirty times more likely.