Getty Images Antibiotics are only necessary for bacterial infections, not viral illnesses like colds or the flu. It's important to finish your entire antibiotic prescription, even if you feel better, to prevent recurrence. Misusing antibiotics can lead to antibiotic resistance, which makes them ineffective against bacteria. Antibiotics treat bacterial infections, such as pneumonia, strep throat, and urinary tract infections (UTIs). They are ineffective against viral infections like the flu.