**Core Concept**
Cryptosporidiosis is an intestinal infection caused by the protozoan parasite Cryptosporidium, leading to symptoms such as diarrhea, abdominal pain, and weight loss. This infection is often associated with contaminated water and food, and can be particularly severe in immunocompromised individuals.

**Why the Correct Answer is Right**
Cryptosporidium parasites invade the intestinal mucosa, causing damage to the microvilli and disrupting the normal absorptive function of the intestine. The parasites also produce toxins that contribute to the pathogenesis of the disease. Importantly, Cryptosporidium oocysts are highly resistant to environmental stressors and can survive for extended periods outside of a host, making them a significant public health concern.

**Why Each Wrong Option is Incorrect**
**Option A:** Cryptosporidium is typically transmitted through fecal-oral contact, which is correct. However, the question asks for "all except," so we'll examine the other options.
**Option B:** While Cryptosporidium can cause disease in immunocompetent individuals, it is particularly severe in those with weakened immune systems, such as HIV/AIDS patients. This option is incorrect because it implies that Cryptosporidium only causes disease in immunocompromised individuals.
**Option C:** Cryptosporidium is indeed a protozoan parasite, which is correct. However, this option is also incorrect because the question asks for "all except."

**Clinical Pearl / High-Yield Fact**
Cryptosporidium oocysts are highly resistant to chlorine disinfection, making water treatment a significant challenge. This resistance is a key factor in the parasite's ability to cause outbreaks of cryptosporidiosis.

**Correct Answer:** A.