illness – enfermedad
(Pronounced ehn-fehr-meh-dad)
To remember that illness is pronounced ehn-fehr-meh-dad (enfermedad) in Spanish, use the following mnemonic:
His father was suffering from a serious illness. He said: 'It's unfair me dad (enfermedad) has to suffer like this'.

Enfermedad is feminine, so it's la enfermedad. Imagine a lady who comes up with a cure for a serious illness:

Examples of enfermedad (illness) in a sentence
Mi enfermedad duró dos semanas en enero.
(My illness lasted two weeks in January.)
El médico dijo que mi enfermedad no es grave.
(The doctor said my illness is not serious.)
Esta enfermedad se puede prevenir con una vacuna.
(This illness can be prevented with a vaccine.)