Illustration of a catalase molecule encapsulated in a nanoparticle. Catalase catalyses the breakdown of hydrogen peroxide to water and oxygen. Hydrogen peroxide is a highly toxic by-product of a number of normal cellular processes and is also often a product of disease. Catalase has been shown to protect lung and nerve cells against damage and to regulate the production of inflammatory immune proteins (cytokines). The overproduction of cytokines, known as a cytokine storm, causes high levels of inflammation that leads to damage and can be fatal, even in those that were otherwise fit and healthy. Catalase nanoparticles could be used as a treatment for Covid-19, the respiratory disease caused by the new coronavirus SARS-Cov-2, that can lead to a cytokine storm in some patients.