I have a fever
A very common, slightly more colloquial alternative, especially in Mexico and Central America. It literally means 'I have heat'.
Uses the verb 'estar' to emphasize the temporary state of having a fever. It means 'I am with fever'.
Means 'I feel feverish'. Use this to describe the feeling rather than stating the medical fact. Use 'afiebrada' if you are female.
I can't go to work today. I have a fever and a headache.
Mom, I think I have a fever. Can you take my temperature?
The doctor asked me if I had a fever.
I've been feeling feverish and very tired since yesterday.
'Tengo fiebre' is the most universal and understood phrase across all Spanish-speaking countries. It is always a safe and correct choice.
'Tengo calentura' is extremely common and used just as much as, if not more than, 'tengo fiebre' in everyday conversation.
'Tengo fiebre' is the standard. 'Tengo calentura' is understood but used much less for fever and can sometimes carry a connotation of being 'hot-headed' or sexually aroused. It's best to stick with 'fiebre'.
'Tengo fiebre' is the standard phrase. Similar to Spain, 'calentura' can have sexual connotations and is best avoided when talking about illness.
Both 'tengo fiebre' and 'tengo calentura' are widely used and understood to mean you have a fever.
Tengo *una* fiebre. — In Spanish, you don't use the article 'una' (a) with 'fiebre'. The correct and natural-sounding phrase is simply 'Tengo fiebre'.
*Soy* fiebre. — A fever is a temporary condition you 'have' (tener), not a permanent characteristic you 'are' (ser). Always use 'tener' or 'estar' (e.g., 'Estoy con fiebre').
Tengo calor. — This means 'I am hot' (due to weather). It does not mean you have a fever. To talk about a fever, you must use 'fiebre' or 'calentura'.