Pathogen virulence refers to the degree of harm a pathogen, such as a bacterium or virus, can cause to its host. It is influenced by various factors, including the pathogen's ability to invade tissues, evade the immune system, and produce toxins. Higher virulence often leads to more severe disease symptoms and increased transmission rates. Different pathogens exhibit varying levels of virulence. For example, Streptococcus pneumoniae can cause mild illnesses like sinusitis but can also lead to severe conditions such as pneumonia or meningitis. Understanding virulence helps in developing effective treatments and preventive measures against infectious diseases.