What are dormant structures that require a host to grow actively?

The correct answer is viruses.

Viruses are unique infectious agents that cannot replicate or grow on their own. They require the presence of a host organism to carry out their life cycle. When a virus enters a suitable host cell, it hijacks the host's cellular machinery to replicate and produce new virus particles. This dependency on a host for reproduction is a fundamental characteristic of viruses, distinguishing them from other biological entities.

Endotoxins are components of the outer membrane of certain bacteria and do not require a host for growth or function. Endospores are highly resistant structures formed by some bacteria to survive harsh conditions; they remain dormant until they find suitable conditions to germinate, but they do not specifically require a host for their life cycle. Probiotics are live microorganisms that can confer health benefits when consumed but do not have a parasitic relationship with a host; they can grow in specific environments like the human gut without the need to hijack host cells.