Which of the following substances is primarily used to treat infections?

Antibiotics are specifically designed to treat infections caused by bacteria. They work by killing bacteria or inhibiting their growth, making them effective for a wide range of bacterial infections. This includes common conditions like strep throat, urinary tract infections, and many more.

The other options, while important in their respective areas of health care, serve different purposes. Analgesics are primarily used to relieve pain rather than to treat infections. Vaccines are preventive measures that help the body build immunity against certain diseases but do not treat infections once they occur. Antihistamines are used to relieve symptoms of allergies and are not effective against bacterial infections. Thus, antibiotics are the clear choice for treating infections.