Final answer:
Infectious diseases are caused by pathogens that enter the body and can be transmitted from one person to another. Pathogens can enter the body through various routes and cause harm by multiplying and damaging the host tissues.
Step-by-step explanation:
Infectious diseases are caused by pathogens, which are bacteria, fungi, protozoa, or viruses that can enter the body and cause harm. These diseases can be spread from one person to another through various routes, such as airborne transmission, direct contact, or fecal-oral transmission. Pathogens can enter the body through openings like the oral cavity, nose, eyes, genitalia, anus, or breaks in the skin. Once inside the body, pathogens multiply and damage the host tissues, causing noticeable symptoms of the infectious disease.