What is the difference between virulent and avirulent?

Virulent refers to pathogens, particularly viruses or bacteria, that can cause severe disease or harm to their host, often resulting in significant symptoms and high transmission rates. In contrast, avirulent pathogens are less harmful or non-pathogenic; they may exist without causing disease or may only lead to mild symptoms. This distinction is crucial in understanding disease severity and the immune response elicited by different pathogens.