Final answer:
Cryptosporidium is difficult to remove from water supplies due to its chlorine-resistant cyst stage, necessitating filtration alongside standard chlorination to ensure water safety, particularly in areas with poor sanitation.
Step-by-step explanation:
Cryptosporidium are difficult to remove from a public water supply because they have a highly resistant cyst stage that can survive the chlorination process traditionally used to treat municipal water. These protozoan parasites create oocysts that are able to withstand harsh environmental conditions, including the chlorine-based compounds often used for sanitizing drinking water. As outbreaks of cryptosporidiosis can occur through contaminated water supplies, additional measures such as filtration must be used to effectively trap and remove these resilient cysts to ensure water safety. The risk of contamination is elevated in areas with poor sanitation, and the disease is more common in developing countries.