Mycobacteria are found in soil and in water, and are typically transmitted to humans by ingestion of microscopic particles of soil existing as dust on carelessly washed or poorly cooked foods, breathed in as dust, or by drinking contaminated water. Most forms of mycobacteria are considered to be very poorly transmissible, and are not easily passed from person to person. Most of the remaining species, however, were previously believed to cause disease only in animals.
Mycobacteria are unusual because they are intracellular parasites .
This is the same strategy so successfully used by HIV to avoid being cleared by the human immune system. In addition to this protection, they are also completely enclosed in a thick waxt cell coat, or cuticle , which effectively resists attack from most treatment drugs. In fact, the mycobacteria are generally almost totally untouchable, except when they are actively reproducing, which is the only time at which they can be killed by treatment drugs. However, because they are so slow-growing, they reproduce fairly infrequently, which means that any treatment given an infected person must be given over a very long time, in order to be certain that it has killed all of the organisms present in an infection. Treatment drugs must also be very strong in order to work properly against these organisms.