Molecular model of the cysteine protease inhibitor K777. Cysteine proteases are enzymes that cleave proteins into smaller molecules. They play an important role in the life cycle of many human parasites and diseases and so molecules that inhibit them are being investigated for use as drugs. K777 has been shown to be effective against a number of parasitic infections, including Chagas disease and schistosomiasis. It has also been shown to block the SARS-CoV-2 virus, the cause of covid-19, from entering human cells and so is a potential treatment for the disease. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), oxygen (red), nitrogen (blue) and sulphur (yellow).