Buruburu is a type of Ghost born from fear, so it strikes fear into its victims - in other words, Ghost sickness.
Characteristics
According to Bobby, the Buruburu are Ghosts born of a person's fear after dying in a terrifying manner. They can then infect others with their fear, and this infection is known as the Ghost Sickness. The people they infect are usually people who have made a habit of terrorizing others. Because it is a type of Ghost, it has all the same abilities and weaknesses as a ghost, but it differs in one notable way: it can be "scared to rest." It is a spirit form first categorized and documented by the Japanese in the Edo period, and the book that Bobby has on the subject is in Japanese.
Season 4
Dean was infected with the Ghost sickness, and Bobby and Sam deduced that the culprit was a Buruburu born from the terrifying nature of Luther Garland's death. Luther was chained to a truck and dragged up and down a gravel road until he died, so it was impossible to find and burn all of his remains. So far, the victims of Luther's ghost sickness have all died of heart attacks brought on by crippling fear, so they had to find another solution quickly.
Luther's spirit is destroyed.
Finally, Sam captured the buruburu with an Iron chain etched with spell work and then had Bobby drag his soul down the same gravel road where he died, re-enacting his death and literally "scaring him to rest." This happened not a moment too soon, as Dean was having hallucinations of Lilith and was close to having a heart attack of his own when they finally managed to defeat Luther's spirit. ("Yellow Fever")