Explanation

Tuberculosis (TB) is an infection caused by three very similar bacteria - Mycobacterium tuberculous, Mycobacterium bovis and Mycobacterium africanum. You can breathe these into your body through your respiratory system. Your immune system, stops you getting infections from bacteria all the time, using white blood cells called neutrophils. But, TB can't be destroyed by neutrophils - your body uses special cells called macrophages to deal with it instead.

TB bacteria are hard to kill because they have some special features, like a strong cell wall. Nearly all TB infection is breathed in through your lungs, so they are affected most by it. TB gets spread by people breathing in the bacteria that someone else has coughed out.

The bacteria travel through your lungs until they get to the alveoli. There, they are swallowed by macrophages in which they start dividing. Within weeks, there are thousands of bacteria in your lungs. Some TB bacteria then go from your lungs to other parts of your body, using your bloodstream or lymphatic system, where they start dividing again.

Your immune system uses T cells to surround the macrophages and form a barricade called a granuloma. The TB bacteria inside the granuloma are walled off from the rest of your body, so as long as your immune system is strong, the bacteria can't escape.

You usually get sick from TB when your immune system has been weakened, like by an infection. TB bacteria can escape through the granuloma and start dividing again. This is called re-activation. HIV can weaken your immune system, and if you have both TB and HIV infection, the HIV infection makes the TB infection worse.

TB is called TB infection when the TB bacteria are in your body, but you're not sick. This is also called LTBI (Latent TB Infection) - latent means the same as hidden. It's called TB disease once you are sick.