Swollen shoot disease of cocoa is caused by a virus. More specifically, it is caused by a group of viruses called Cocoa swollen shoot virus (CSSV). These viruses are transmitted to cocoa plants by mealybugs, which are small insects that feed on the sap of the plant. The virus then infects the plant, causing its shoots to swell and become distorted. Over time, the infected plant becomes stunted, and its yield is significantly reduced. The disease is a major problem in many cocoa-growing regions, and there is currently no cure for it.