# Phase Therapy: Consider the Past, Embrace the Future
## 1 Introduction  
Since [PERSON] and [PERSON] discovered bacteriophages in 1915, they have become a research hotspot as a natural antimicrobial agent. [PERSON] attempted to treat dysentery with phage in 1919 [1]. However, due to the discovery and application of broad-spectrum antibiotics in the 1940s, diseases caused by bacterial infections can be effectively treated, and the morbidity and mortality rates have dropped significantly; therefore, the phage therapy was quickly dwarfed [2]. Eastern Europe became the only region in the world that viewed phage therapy as an alternative to antibiotics. Among them, Poland and the Soviet Republic (particularly the Georgia and Russia Soviet Socialist Republic) have made significant contributions to the development of phage therapy because of their insistence [3].  
In the war against pathogenic bacteria, antibiotics have always been an important weapon in human hands. The discovery and use of antibiotics have made great contributions to modern medicine. However, a series of problems have become increasingly serious with the extensive use even abuse of antibiotics. First, the massive use of antibiotics leads to bacterial resistance mutations. With the continuous accumulation and spread of bacterial resistance, some pathogens have become superbugs since no antibiotics can fight against them [4]. According to the \"Antibiotic Resistance Threats In The United States\" reported by the US Centers for Disease Control and Prevention in 2019, more than 2.8 million cases of drug-resistant bacterial infections occur in the United States each year, resulting in more than 35,000 deaths. In addition, due to the broad-spectrum and high-efficiency killing ability of antibiotics, antibiotic therapy is often accompanied by the risk of harming human health. For example, antibiotics can affect gut microbes and destroy intestinal homeostasis [5,6]. In addition, bacteria develop resistance in an increasingly fast manner, compared with the slowprogress of new antibiotics development over the past few decades [7]. Therefore, the development of highly effective and safe antimicrobials is particularly important. Bacteriophages have once again attracted researchers' attention since they can specifically lyse bacteria and are not likely to cause bacterial resistance compared to traditional antibiotics because of their selective killing ability and co-evolution with host bacteria. It is an effective method to replace antibiotics and treat bacterial diseases. Some attempts to use bacteriophages against pathogenic bacteria in animal breeding, plant disease prevention, food manufacturing, and so on, have been reported [8; 9; 10]. So far, there have been successful cases of applying bacteriophage to the treatment of human drug-resistant bacterial infections [11; 12; 13]. With the development of research, the clinical application of bacteriophages should be more normalized and extensive.  
This review mainly introduces the principle of phage therapies for bacterial infection, elaborates on the advantages of phage therapy over traditional antibiotic treatment, and summarizes the current common approaches for phage therapy. In addition, the possible obstacles, risks, and application prospects of phage therapy are discussed.