Numerous factors have been identified that account for the resurgence of tuberculosis in the United States and in Europe as well. The rapid response of state and federal agencies in the United States has averted a potentially drastic rise in incidence. The cell wall's structure and function are not well understood but appear to allow the bacteria to survive within immune cells called macrophages, specialized cells that destroy bacteria and viruses. It also provides the organism with a resistant barrier to many common drugs. The organism is difficult to study in the laboratory.
Slow growth makes culturing a lengthy process and colony formation can take several weeks.
TB bacilli form clumps, making them difficult to work with and to count. Because this is a dangerous airborne pathogen, their study requires special safety equipment. The bacteria's primary host is the human. When an infectious person talks, coughs, sings, or spits, tiny aerosolized droplets containing a few bacteria are spewed into the air and inhaled into the lungs of an uninfected person. Viable bacteria can remain in the air for a long time.