Once considered punishment from a higher authority, we now understand that leprosy is actually an acquired chronic bacterial infection caused by Mycobacterium leprae (M. leprae). It's contagious — between two humans, yes, but it's also transmitted from armadillo to human. Because skin lesions are visible they are the distinguishing characteristic of the disease, but it's the damage to peripheral nerves that is at the core.
No one is 100 percent sure how you become infected with M. Laprae, but it's widely accepted that among humans it's transmitted through the small droplets that are released with a cough and sneeze — a sneeze, for instance, can disperse those droplets as fast as 10 miles (16 kilometers) per hour [source: Engber]. That might not be fast for a car, but it's definitely a good clip for something flying from a respiratory tract.
M. Leprae is a parasitic organism, which means it relies on those host cells for its survival, and it's kind of tricky once it gets into your body. This class of bacteria target your Schwann cells — those are nerve cells that the body uses to fix its peripheral nervous system — and changes the way they work. First, the organism hides in these cells, which means your immune system doesn't see them, and the immune system won't fight what it doesn't know is there. It may take years for the disease to appear after you've been infected, but inside your body the infected cells remain under attack. As the infection takes hold, the cells begin to break down, and once they lose their protection M. Leprae exploits them. Bacilli-infected Schwann cells are converted into cells that behave like stem cells, and here's what makes that so deadly. Because stem cells have the ability to convert into another type of cell in your body, imagine the power this gives to the organism. If an infected Schwann cell is converted to a mycocyte (a muscle cell), for instance, M. Leprae is now infecting the body's muscle tissue. And as nerve fibers are affected, symptoms of the disease begin to appear.
Skin lesions, which might be flat or raised, and might appear solo or in groups, are the hallmark of leprosy. In more severe cases, the infection may cause respiratory problems ranging from hoarseness to nasal symptoms (including loss of sense of smell, stuffy nose, bleeding, and even a collapsed nose). Eyes are also at risk for damage, and leprosy may cause eye redness, eyebrow and eyelash hair loss, pain, lagopthalmos (which is what it's called when you can't close your eyelids), and other eye ailments, all leading to blindness.
Additionally, and most significantly, M. leprae causes nerve damage. Trophic ulcers (commonly on the plantar area on your feet) may appear along with related loss of pain and skin sensation. Motor impairment, as well as muscle weakness and paralysis (including such problems as claw-hand deformities, facial nerve palsy) also disable leprosy sufferers.