Ernest Price, a British surgeon, figured out in the 1970s that podoconiosis was caused by years of barefoot exposure to red clay soil [link].
Which particle or mineral within this soil causes the problem is still being investigated: it may be very tiny grains of silica that are responsible, or some other soil component associated with silicate rich soil.
These particles enter the body through bare skin, causing inflammation which in turn forms blockages known as lymphoedema. The skin becomes rough and bumpy (looking like moss, hence the name) and, with recurrent infection, produces an offensive smell. The affected limb becomes swollen with fluid: this is painful and often the discomfort, combined with the sheer size, weight and impaired function of limbs, makes it impossible for the sufferer to walk, let alone carry out their normal work and family duties. Sufferers are frequently ostracized, forced to become beggars and many believe they have been cursed.