Which statement describes a necessary step for a pathogen to cause infection?

• A. Exit the host before entering.
• B. Enter the host body and adhere to specific host cells.
• C. Travel through water only.
• D. Infect only plants.

Answer: (B)

A pathogen must get inside the host and attach to specific host cells to establish an infection. Entering the body gives the microbe access to tissues and nutrients it needs to multiply, while sticking to host cells prevents it from being washed away and positions it to interact with receptors that can enable invasion or damage. This adhesion step is often essential for establishing a foothold, whether a bacterium uses pili or an virus binds to cell receptors to gain entry. Without entering, the microbe remains outside and cannot cause an infection; without adhering, it would be cleared by the body's defenses. The other statements don’t describe a necessary step for infection in a host.