Disease
The body being a structure, an object, is subjected to impermanence and decay. In addition to the gradual weathering (aging, as discussed), a body suffers damages suddenly due to various agencies and reasons. The harm can come from external agents such as microbes or from internal causes such as organ failures or from mental influences such as stress and anxiety.
These days it is easy to cure the diseases caused by external agencies, because of their purely physical nature. There is a good understanding about the nature of microbes and medicines that kill them. Still people die from pathogenic infections in millions. The reasons are social and Egoic, not a lack of knowledge and resources. Anyhow the progress in this regard is extraordinary, when you compare the situation today with ancient days, when an infection meant a certain death. The body has built in defences against microbes, else it wouldn't survive for even one day, but sometimes the pathogens find a way in. Needless to say that it is of prime importance to keep this immune system in good shape, all one needs to do is not to damage it intentionally. One can damage it by the means of substance abuse or a shabby and unhygienic lifestyle or by maintaining too much cleanliness. By too much cleanliness, I mean the indiscriminate use of chemicals that kill microbes inside or outside the body. Immune system constantly needs stimulants in order to maintain its defences, and in the absence of any microbes it becomes inactive. So a good rule of thumb is to avoid dirty surroundings, stale food, harmful substances and operation theater quality cleanliness.
