Which pathogen MUST live in or on a host to survive?

The correct response highlights Cryptosporidium parvum as a pathogenic organism that requires a host to survive. This microorganism is a protozoan that specifically infects the intestinal tract of humans and animals. It cannot live outside a host for extended periods, as it relies entirely on the host's biological systems for replication and transmission.

The life cycle of Cryptosporidium includes the production of oocysts, which can be excreted in stools of infected hosts and can contaminate water or food sources. However, these oocysts are just a part of its life cycle; the organism itself must inhabit a living host to thrive and complete its lifecycle.

In contrast, other options, such as Norovirus, can survive for a time outside of a host, particularly on surfaces or in contaminated foods, meaning it doesn’t rely exclusively on a host for its survival. Clostridium perfringens has spores that can persist in the environment, enabling it to survive without a host. The distinction of Cryptosporidium parvum emphasizes the necessity of a host for its life processes, making it unique among the listed pathogens.