Phages are the natural enemies of bacteria. They only attack bacteria and are harmless to people, animals, and plants. Utilizing this property of phage, phage therapy is proposed to treat bacterial infections. The first use of phage therapy traces back almost a century ago, which is earlier than antibiotics application. Phage therapy has received relatively little attention due to the powerful antibacterial effects of antibiotics. But with the rapid emergence of multiple antibiotic-resistant (or multidrug-resistant, MDR) bacteria, there is renewed interest in using phages as therapeutic agents.