BBC | Phages or bacteriophages are viruses that naturally prey on bacteria by infecting and replicating within them until they burst out, killing their microbial host. There are billions of phages on Earth, and they have co-evolved with the bacteria they prey on for millennia, helping to keep their numbers in check. Their therapeutic use was first pioneered in 1919 by Felix d’Herelle, a French-Canadian microbiologist who used phages to cure a boy suffering from severe dysentery. However, the discovery of penicillin in 1928 and its subsequent commercial production by the 1940s unleashed the antibiotic era, effectively supplanting phage therapy. Phage is attracting attention again in the search for better ways to address drug-resistant bacteria. But there remain plenty of challenges, including regulatory obstacles that need to be addressed.