I have a fever.
FEB-bre — double 'b', short and crisp. The 'e' in 'febbre' is open.
The most common phrase when describing illness. Say it at the doctor's, to a pharmacist, or when calling in sick to work.
'Ho la febbre' uses 'avere' (to have) with the definite article 'la'. Italians say 'ho la febbre' not 'ho febbre' — the article is obligatory here. In Italy, a fever is typically measured in Celsius: 37°C is normal, 37.5°C is a low fever ('febbricola'), 38°C+ is a fever.
Ho trentotto di febbre.
I have a fever of thirty-eight degrees.
Specify the exact temperature — Italians always use Celsius
Ho i brividi.
I have chills.
Often accompanies fever — 'brividi' are shivers or chills
Mi sento caldo e stanco.
I feel hot and tired.
Describe associated symptoms
In Italy, body temperature is taken rectally for babies, but orally or under the arm for adults. Under-arm readings are about 0.5°C lower than oral. The traditional thermometer was glass and mercury — now digital. Fever above 39.5°C in adults or 38°C in infants should be seen promptly at 'pronto soccorso' (A&E).