Although the words afflict and inflict have similar meanings and are often used in similar ways, they are far from interchangeable in modern English usage. The OED offers the following definitions of afflict:
1. trans. To dash down, overthrow, cast down, deject, humble, in mind, body, or estate.
2. intr. To become downcast (with trouble).
3. trans. To distress with bodily or mental suffering; to trouble grievously, torment. refl. To distress oneself, grieve.
In the case of afflict the third definition is the closest to the most common use of the word. The emphasis is on the physical or psychological distress caused by whatever the affliction may be. The word afflict is most commonly used when describing an illness or condition. For example:
He was severely afflicted with gout.
She suffers from a terrible affliction.
These sentences demonstrate an accurate use of the word as they describe a form of physical distress.
Inflict is defined by the OED as follows:
1. trans. To lay on as a stroke, blow, or wound; to impose as something that must be suffered or endured; to cause to be borne.
2. To impose something unwelcome. (Often jocular).
The emphasis of the word inflict is upon the imposition, the force and the unwelcome nature with which whatever is being inflicted upon a person is being inflicted. For example:
A severe punishment was inflicted on the hooligan.
The teacher inflicted a thirty minute detention upon the rowdy pupil.
This is an accurate use of the word as it concentrates not on the punishment and the distress it causes but the force with which the punishment was administered.
It is of little surprise that these two words are so regularly confused as the OED mentions the word affliction in a further definition of the word inflict as follows:
With inverted construction: To afflict, assail, trouble (a person) with something painful or disagreeable.
An additional note added to this strand of the definition explains that this is now a rare use of the word inflict. Traditionally the words were more interchangeable but in modern English they are clearly very separate entities. When in doubt ask yourself whether or not you are talking about something somebody is doing to another person – inflict or whether you are talking about something with which somebody is suffering and the distress it is causing them – afflict. This will enable you to use each of the words in the most commonly understood terms.