Which organism is the hardest to kill among the following?

Cryptosporidiosis, caused by the parasite Cryptosporidium, is known for its resistance to many common disinfection methods used in water treatment. Unlike bacteria and viruses that can often be effectively eliminated through chlorine disinfection, Cryptosporidium is protected by a tough outer shell, making it highly resilient in the environment.

This resistance is particularly concerning because it can survive in chlorinated water and is also resistant to other chemical disinfectants. Additionally, it requires higher levels of chlorine and longer exposure times to be effectively killed, which is not always feasible in practical water treatment scenarios. Therefore, it is crucial for water treatment facilities to implement multiple barriers and methods for effective removal or inactivation of Cryptosporidium, reinforcing its reputation as one of the hardest organisms to eliminate in water treatment systems.