It does not mean that the person literally catches fire. Physical illness and mental illness are similar in that both are undesirable states which may lead to suffering, disability or death. The difference between them is that physical illness refers to undesirable states of the body and is studied and treated by the methods of the physical sciences. Mental illness refers to undesirable states of mind, mood and behavior and is studied and treated through language, communication, interpretation, evaluation, persuasion and social manipulation. Every society in history has believed as true, myths which other societies, of different places or time, regard as wishful beliefs, fairy tales, or superstitions.
Modern myths depict mind as a manifestation of brain physiology.
Primitive and modern myths are basically similar. They both confuse logical categories. The difference between them is just that they move in opposite directions. Logically, the belief that mental illness is fundamentally similar to physical illness is like believing that hurricanes are caused by angry gods, or that a person with a fiery temperament actually bursts into flame. Becker's early psychiatric education was based on Szasz's critical analysis of psychiatric thought and practice.