How serious is "contact lens eye infection" really?

Two very different things often called "infection":

• Mild conjunctival irritation: redness, mild itch, slight discharge. Usually resolves within 48 hours of removing lenses and using preservative-free artificial tears. Many causes — solution sensitivity, mild allergy, dry eye.
• Microbial keratitis: bacterial, viral or parasitic infection of the cornea itself. Pain, severe redness, light sensitivity, vision change. Can permanently scar the cornea within 24–72 hours if untreated. Treatment is intensive topical antibiotics (sometimes hourly) for 1–4 weeks.

Risk factors that raise microbial keratitis above the baseline:

• Sleeping in standard contact lenses (6–8× higher risk).
• Showering or swimming in lenses without goggles.
• Using tap water for lens or case rinsing.
• Skipping the rub-and-rinse step.
• Wearing lenses past their replacement schedule.
• Smoking.

Absolute risk for a careful lens wearer is roughly 4 per 10,000 per year. Daily disposables roughly halve that. The single biggest reduction is "don't sleep in your lenses".