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Ascending weakness after a diarrheal illness points to a peripheral neuropathy driven by an autoimmune attack on the nerves. Campylobacter jejuni infection is a classic trigger for Guillain-Barré syndrome, an acute inflammatory demyelinating polyneuropathy. The body's immune response to the bacterium cross-reacts with components of peripheral nerves (mopathy via molecular mimicry), leading to demyelination and sometimes axonal injury. Clinically, this produces rapidly progressive weakness that begins in the legs and ascends, often with decreased or absent reflexes. Sensory symptoms and autonomic changes can occur, and CSF may show high protein with normal cell count after the first week or two. This pattern distinguishes it from other conditions: multiple sclerosis affects the central nervous system with different presentation and timing; myasthenia gravis causes fatigable weakness that worsens with use and typically involves eyes or bulbar muscles; systemic lupus erythematosus can involve nerves but does not classically present as an ascending post-infectious neuropathy.