Why is TB contagious?

Chris - Well, tuberculosis is a bacterial infection. It's an infection caused by Mycobacterium tuberculosis. There's also another form called Mycobacterium bovis, which is actually as the name suggests carried by cows but can occasionally get into humans. And people who have TB and specifically open TB, in other words they're actively infected with it, they have TB growing in their lungs, they're infectious. And these bacteria are absolutely tiny. There are many of them and once a person has TB they can remain infected with it for very long periods of time. And about one-third of the world's population is thought to be infected so that's something in the region of two billion people on Earth who've got TB and those numbers are increasing. The death toll because of TB is in the thousands every single day so it's a very major disorder, and it's a very common disorder because it's a very infectious disease. So if you have infectious cases, then you will get onward spread. And it used to be a major problem until we did things like testing for it and then vaccinating. We give people the BCG which is a disabled live bacterium, a mycobacterium which you put into someone and this educates the immune system to help them to react to TB. And this reduces the chances of you getting a serious dose of TB in the future.