Pneumonia is usually caused by a lung infection. COVID-19 is one type of pneumonia, caused by infection with the SARS-CoV-2 virus. Other viruses, like the flu or respiratory syncytial virus, cause similar types of viral pneumonia. Bacteria are also important causes of pneumonia, with pneumococcus (Streptococcus pneumoniae), being most common. The outcome of pneumonia is determined by the interactions of microbes and microbial products with the body. Additional pages within this “Background” section provide general information about pneumonia, designed to be of interest regardless of scientific or medical training.