Giardia exists in two forms--cysts and trophozoites. While both are required for the life cycle of Giardia, only the cyst is considered infectious. Why is the cyst considered the infectious form of Giardia

Because the trophozoites are unable to survive outside a host.

Explanation:

The term infectious is used for a disease or microorganism that can be transmitted from one organism to the other organism through the environment. If we look at the two forms of Giardia, we find that out of the two forms viz. cysts and trophozoites, only the cysts of Giardia have the ability to remain alive in the environment outside the host and thus they are able to be transmitted to the other organisms. On the other hand trophozoites are not able to survive in outside environment so there is no chance of transmission of trophozoites.