Helicobacter pylori is a gram-negative, microaerophilic, spiral-shaped bacterium colonizing ~50% of the world's gastric mucosa. It produces urease converting urea to ammonia, creating an alkaline microenvironment for colonization. H. pylori causes chronic gastritis, peptic ulcer disease (90% duodenal, 70% gastric ulcers), gastric MALT lymphoma, and gastric adenocarcinoma (WHO Group 1 carcinogen). Eradication is indicated for: active PUD, history of PUD, MALT lymphoma, post-gastric cancer resection, uninvestigated dyspepsia <60 without alarm features, chronic NSAID users, and unexplained IDA or ITP. First-line depends on local clarithromycin resistance: if <15%, triple therapy (PPI + clarithromycin + amoxicillin × 14 days); if ≥15% or unknown (most US settings), bismuth quadruple therapy (PPI + bismuth + metronidazole + tetracycline × 14 days) or concomitant therapy (PPI + clarithromycin + amoxicillin + metronidazole × 14 days). Confirm eradication ≥4 weeks after therapy using urea breath test or fecal antigen (NOT serology — antibodies persist). Hold PPIs ≥2 weeks and antibiotics ≥4 weeks before testing.