Final answer:
Pathogens must use a portal of exit to spread from an infected host to a new one, commonly through the skin, respiratory, urogenital, and gastrointestinal tracts via secretions, excretions, and other methods like insect vectors.
Step-by-step explanation:
To cause disease and spread to new hosts, a pathogen must leave the host through a portal of exit. The most common portals of exit include the skin and the respiratory, urogenital, and gastrointestinal tracts. Coughing and sneezing, for instance, expel pathogens from the respiratory tract. Other methods include secretions and excretions, such as feces, urine, semen, vaginal secretions, tears, sweat, and sloughing off skin cells. Some pathogens also leave the body in blood drawn by insects or needles, utilizing these as vectors for transmission.
Each pathogen is generally adapted to exit from a particular portal, which is determined by its environmental adaptations and the biological machinery it possesses. For survival and propagation, a pathogen's ability to exit one host and successfully infect a new host is as crucial as its ability to cause the initial infection.