Kuru Disease
From "Mad Cow Disease" by Mark Purdey

Similar to CJD, kuru is a progressive neurologic disorder that occurs primarily in the Fore natives who inhabit a tiny pocket of the New Guinea highlands. Symptoms are much like vCJD and include an exaggerated startle response and emotional instability, with pathologic bursts of laughter. Advanced states are characterized by dementia. In the terminal state, the patient is generally totally placid, mute and unresponsive.

Until the early 1960s, the disease was prevalent, especially in women and children, but in recent years the incidence has declined. This was said to be due to the abandonment of ritual cannibalistic practices in which natives ate the flesh of the dead, a theory that fit in very well with the dogma that the bovine version was caused by consumption of infected meat and bone meal. But the entire native population across New Guinea were traditionally involved in cannibalism, so why Kuru in just one tiny region? A more likely explanation is the massive eruption of a local volcano in 1911 which showered the foodchain of the Fore region in a black manganese oxide ash—the decline in Kuru paralleling the importation of more foodstuffs from the outside world. Both the early stage unmotivated laughter with psychosis, and the placid, unresponsive characteristics of advanced Kuru are similar to the symptom profile of manganese poisoning.