For millennia thought to be a curse or mysterious plague, leprosy, also known as Hansen's disease (HD), is a curable infection caused by the bacterium Mycobacterium leprae. It's a chronic, infectious condition that can survive unnoticed and untreated in a subject for as many as forty years. Method of transmission is unsure, but the bacteria is not aerosol, or transmitted without physical contact, with the possible exception of the inhalation of dust containing the bacterium.
Leprosy has long since been associated with unhygienic living conditions, but this association is not wholly reasonable, as poor hygiene only contributes to the contraction and spread of the disease--some sample of the bacterium must be present in an environment for it to be contracted, so naturally crowding is a factor in its transmission as well. Poor nutrition, also, can lead to immune deficiency, which can exacerbate the spread of the condition.
The symptoms exhibited by victims of leprosy vary, depending upon the classification of the condition, though both are caused by the same bacterium. The two main such classifications are lepromatous and tuberculoid cases, both of which represent different sides of a symptomatic spectrum, rather than separate disorders. Lepromatous leprosy is the more major form of the disease, much more generalized than tuberculoid, and often incorporates organs and peripheral nerves that are not usually associated with the milder form of the disease.
Patients of lepromatous leprosy are home to a much greater host of bacteria, and have the potential to suffer severe and permanent nerve damage, if the condition goes untreated. Tumors may be prevalent upon the skin. Tuberculoid patients, however, experience only the appearance of minor, pale or reddish dry lesions which are usually flat or slightly raised, and which are insensitive to the touch.
Subtypes include borderline tuberculoid, midborderline, and borderline lepromatous, depending upon the extent and severity of the disorder. A single lesion indicative of potential leprosy is usually referred to as indeterminate leprosy, but has potential to progress to more serious forms of the condition. An alternative, more recent classification system simplifies the typification of leprosy into only Paucibacillary (PB) and Multibacillary (MB), the former being the less significant (tuberculoid) and the latter being the more serious (lepromatous). If treated for leprosy, your physician may use either set of terminology, likely depending upon his or her age.
Because it is, by bacterial standards, a hardy infection, multiple drugs should be used to treat both the MB and PB forms of leprosy. The most common group for MB leprosy is clofazimine, dapsone, and rifampicin. PB is usually approached with only rifampicin and dapsone. Resistant cases have been reported, but are exceedingly uncommon.
Historically, lepers were feared to such a degree that they were usually cordoned off into "colonies" to prevent the spread of the disease, even up until the late nineteenth century. It may be of interest to note that for many centuries after the Black Death of the 14th century, leprosy was not a condition known to Europe, as all those with the condition died off during the course of the plague. This was likely due to the close living conditions associated with whatever isolation they may have endured, as well as that the poor immune health associated with the contraction of leprosy would doom most anyone to the plague, should it come around. Leprosy returned to Europe some centuries later as a gift of the crusades.