Which biological pathogen is known for its high rate of secondary transmission?

The pathogen known for its high rate of secondary transmission is actually Norovirus. This virus is highly contagious and can quickly spread through contaminated food, surfaces, or direct contact with an infected person. Its ability to cause outbreaks, especially in closed environments like cruise ships, schools, or nursing homes, is due to the ease with which it can be transmitted from one person to another, often with minimal exposure.

Influenza, while also easily transmissible, does not match the rapid spread seen with Norovirus. Smallpox was eradicated in the late 20th century through vaccination, and while it had high transmission rates prior to that, it is no longer a factor today. HIV primarily spreads through specific risky behaviors and does not have the same level of secondary transmission as Norovirus. Thus, Norovirus is the correct answer due to its high contagiousness and the ability for outbreaks to occur swiftly in affected populations.