Microbes outnumber human cells 10 to 1 and play important roles in human health and disease. While some microbes cause illness, most are harmless or beneficial. Microbes are found everywhere including in the air, water, soil, and inside and on the human body. They aid in digestion and prevent infection. However, overuse of antibiotics has led to increased antibiotic resistance in microbes, threatening our ability to treat infectious diseases. Antibiotic resistance occurs as bacteria evolve and share DNA allowing them to survive drug exposure. Prudent antibiotic use is needed to slow the development and spread of resistant microbes.