What is a presumption of abuse and how does it affect the case?

When a chapter 7 case is filed by an ineligible person, under bankruptcy terminology that person is said to have abused the chapter 7 laws.  When a person whose current monthly disposable income is such that he or she can afford to make monthly payments to unsecured creditors in the required amount, a presumption of abuse is said to arise in the case.  If a presumption of abuse arises in a case, the case will be dismissed or converted to chapter 13 unless the person filing the case can prove the existence of special circumstances, such as a serious medical condition.