Sick
A general way to say 'I feel unwell' or 'I feel bad'. It's very common and slightly less specific than 'enfermo'.
Use this specifically when you have a cold.
Use this to say 'I have the flu', which is typically more severe than a cold.
Use this to say 'I feel dizzy' or 'I feel nauseous'.
I am not going to school today. I'm sick.
My grandmother feels a little unwell.
Are you sick, ma'am? Do you need a doctor?
I think I have the flu. I need to rest in my bed.
While 'estar enfermo' is perfectly correct, it is very common to hear 'estar malo' or 'estoy malo/a' to mean 'I am sick'.
'Estar enfermo' is standard. For an upset stomach, it is common to hear 'estar malo del estómago' or 'sentirse malo del estómago'.
'Estar enfermo' is used, but for stomach sickness or feeling generally unwell (especially from food), 'estar descompuesto/a' is much more frequent.
'Estar enfermo/a' is the most standard and common phrase. 'Me siento mal' is also used very frequently for a general feeling of illness.
Using 'ser' instead of 'estar'. For example: 'Soy enfermo.' — Always use 'estar' for temporary conditions like being sick: 'Estoy enfermo.' 'Ser' is for permanent traits. Saying 'Soy enfermo' implies you are a sickly person by nature, not that you are sick right now.
Not matching the gender. For example: 'María está enfermo.' — Adjectives must match the gender of the person. For a female, you must use the feminine form: 'María está enferma.'
Translating the English slang 'sick' (meaning 'cool'). For example: '¡Ese carro es enfermo!' — In Spanish, 'enfermo' only ever refers to illness or something disturbing. It never means 'cool' or 'awesome'. To say something is cool, you would use words like 'genial', 'chévere' (Latin America), or 'chido' (Mexico).