Final answer:
A pathogenic bacteria or virus that enters the body and disrupts normal function causes an infectious disease. The virulence of the pathogen affects the severity of the disease, and infections can be local or systemic based on the pathogen's spread within the body.
Step-by-step explanation:
A pathogenic bacteria or virus entering the body and multiplying to the point of interfering with the body's normal state causes an infectious disease. Infectious diseases are illnesses resulting from an infection where pathogenic microorganisms such as bacteria and viruses enter the body, multiply, and cause damage to the host tissues.
The process by which a pathogen causes disease is known as pathogenesis and includes stages such as exposure, adhesion, invasion, infection, and transmission. Infections can be local, focal, or systemic, depending on the spread of the pathogen in the body. A systemic infection indicates that the pathogen has spread throughout the body, while a local infection remains confined to a small area. Virulence is the pathogen's ability to cause disease, which may vary from one organism to another, ultimately affecting the severity of disease symptoms.
Pathogens with high virulence can lead to severe illness or even multi-organ failure, while those with low virulence might only cause mild symptoms or be asymptomatic.