Tuberculosis is caused by a bacterium, Mycobacterium tuberculosis. Most
US cases come from it, though other mycobacteria — such as *Mycobacterium
bovis* — can also cause TB disease in people.
TB usually affects the lungs, and can also affect the **brain, kidneys or
spine**, or several parts of the body at once — the lungs and the lymph
nodes together, for example.
The distinction everything rests on
**Not everyone infected with TB germs becomes sick. As a result, two
TB-related conditions exist: inactive TB (latent TB infection) and active TB
disease.**
One organism, two states:
| Inactive TB | The bacteria are present; the person is not ill and not infectious. Treatable, to prevent progression |
| Active TB disease | The person is ill, and — if it is in the lungs — infectious |
Almost every practical question about tuberculosis resolves differently
depending on which of the two you are talking about: whether contacts need
tracing, which drugs are used, how long for, and whether anyone else is at
risk.
Confusing the two is also dangerous in a specific way: treating someone who
actually has active disease with the shorter, lighter regimen meant for
inactive infection is how drug resistance is made.
Why to get tested
If not treated properly, TB disease can be fatal.
**Getting tested and treated protects yourself, your family and friends, and
your community** — the last of which is the part that applies even when you
feel entirely well, because inactive TB found today is active disease
prevented later.
Source: Centers for Disease Control and Prevention.
许可协议： CC0 1.0（公有领域） · 改编自 www.cdc.gov
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