Infectious disease is defined as

Infectious disease is defined as

• A. Any disease that is caused by an agent that can pass from one living thing to another
• B. A disease that can only be transmitted by direct contact
• C. A disease caused by a bad diet
• D. A disease with no known cause

Answer: (A)

Infectious disease is defined by the presence of a pathogen—such as a bacterium, virus, fungus, or parasite—that can be transmitted from one living organism to another. The crucial idea is that the disease involves a cause that can move between hosts, not just a condition arising from nonbiological factors. Transmission can happen in many ways: through direct contact, sneezing or coughing droplets, airborne spread, contaminated food or water, or via vectors like ticks and mosquitoes. This full view captures why infections can spread even without close or direct touch. The other options don’t fit as well. A disease that can only be transmitted by direct contact is too narrow because many infections spread through the air or contaminated surfaces. A disease caused by a bad diet isn’t infectious because nutrition-related conditions aren’t caused by pathogens and aren’t spread between people. A disease with no known cause isn’t a defining feature of infectious diseases, since many infectious diseases have identifiable pathogens.

Infectious disease is defined by the presence of a pathogen—such as a bacterium, virus, fungus, or parasite—that can be transmitted from one living organism to another. The crucial idea is that the disease involves a cause that can move between hosts, not just a condition arising from nonbiological factors. Transmission can happen in many ways: through direct contact, sneezing or coughing droplets, airborne spread, contaminated food or water, or via vectors like ticks and mosquitoes. This full view captures why infections can spread even without close or direct touch.

The other options don’t fit as well. A disease that can only be transmitted by direct contact is too narrow because many infections spread through the air or contaminated surfaces. A disease caused by a bad diet isn’t infectious because nutrition-related conditions aren’t caused by pathogens and aren’t spread between people. A disease with no known cause isn’t a defining feature of infectious diseases, since many infectious diseases have identifiable pathogens.