Virulence refers to the degree of pathogenicity or harmfulness of a microorganism, such as a bacterium or virus. It indicates how effectively a pathogen can cause disease in a host. Factors influencing virulence include the pathogen's ability to invade tissues, evade the immune system, and produce toxins. Different pathogens exhibit varying levels of virulence. For example, Streptococcus pneumoniae is known for its ability to cause pneumonia, while Escherichia coli can lead to foodborne illnesses. Understanding virulence helps in developing treatments and preventive measures against infectious diseases.