**Core Concept**
_Helicobacter pylori (H. pylori)_ is a Gram-negative, spiral-shaped bacterium that is a major cause of gastric infections worldwide. It is a significant pathogen that can lead to various gastrointestinal disorders, including gastritis, peptic ulcers, and gastric cancer.

**Why the Correct Answer is Right**
The correct answer is related to the association between _H. pylori_ infection and gastric mucosa-associated lymphoid tissue (MALT) lymphoma. Studies have shown that eradication of _H. pylori_ through antibiotic treatment significantly reduces the risk of developing gastric MALT lymphoma. This is because _H. pylori_ infection leads to chronic inflammation in the stomach, which can eventually result in lymphoid proliferation and MALT lymphoma. Eradication of _H. pylori_ interrupts this process and prevents the development of lymphoma.

**Why Each Wrong Option is Incorrect**
**Option A:** H. pylori is indeed a Gram-negative bacterium, but the question is asking for a true statement about the organism. This option is incorrect because it is a factual description of the bacterium, not a statement about its pathogenesis or treatment.

**Option B:** H. pylori is actually a spirilla, not a protozoa. Protozoa are single-celled organisms that belong to the kingdom Protista, whereas H. pylori is a bacterium that belongs to the domain Bacteria.

**Option C:** While it is true that _H. pylori_ infection can lead to chronic gastritis, the statement that chronic gastritis in adults is due to reinfection is incorrect. Chronic gastritis in adults is more likely due to persistent infection rather than reinfection.

**Clinical Pearl / High-Yield Fact**
It is essential to note that _H. pylori_ infection is a significant risk factor for gastric cancer, and eradication of the bacterium can reduce this risk. The "Correa cascade" describes the progression from chronic gastritis to atrophic gastritis, intestinal metaplasia, dysplasia, and eventually gastric cancer, with _H. pylori_ infection playing a key role in this process.

**✓ Correct Answer: D. Treatment prevents gastric lymphoma**