**Core Concept**
Helicobacter pylori (H. pylori) is a gram-negative, spiral-shaped bacterium that primarily inhabits the stomach lining, causing chronic gastritis, peptic ulcers, and an increased risk of gastric cancer. The bacterium's pathogenicity is linked to its ability to colonize the gastric mucosa and evade the host's immune response.

**Why the Correct Answer is Right**
Option C is incorrect because it is not true that all children in developing countries have immunity to H. pylori by five years of age. In reality, H. pylori infection is highly prevalent in developing countries, where children are often exposed to the bacterium during early childhood. However, immunity is not a guaranteed outcome, and many children remain infected throughout their lives. The bacterium's ability to evade the immune system is a key factor in its persistence and the development of associated diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while it is true that a significant portion of the global population is infected with H. pylori, the actual prevalence is estimated to be around 50-60%, not 50%. However, this option is close enough to be a plausible distractor.

**Option B:** This option is incorrect because while H. pylori infection is indeed more common in developing countries, the prevalence is not as high as 85%. The actual prevalence varies widely depending on the specific country and population being studied.

**Option D:** This option is correct, and we're looking for the incorrect option, but it's still worth noting that H. pylori infection is indeed more common in individuals with lower socio-economic status, who may have less access to clean water, sanitation, and healthcare.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that H. pylori infection is a significant risk factor for gastric cancer, and early detection and treatment can help prevent this outcome. The bacterium's ability to evade the immune system makes it a challenging target for treatment, but eradication therapy can be effective in many cases.

**✓ Correct Answer: C. All children in developing countries have immunity by five years of age**