The short answer
Cyberchondria is a term researchers use to describe how searching for symptoms online tends to escalate health anxiety rather than resolve it. It's not an official diagnosis — it's a studied behavioral pattern, first named and researched by Microsoft researchers analyzing how people's search behavior changed after symptom searches.
The research
Researchers coined the term 'cyberchondria' to describe how searching symptoms online tends to escalate — not resolve — health anxiety, since search results are weighted toward rare, severe conditions rather than common, benign ones.
— White & Horvitz, Microsoft Research, 2009
Why search engines make it worse, structurally
Search results for symptoms are disproportionately weighted toward rare, severe conditions — partly because rare conditions generate more written content (case studies, forum posts, alarmed discussion) than common, benign explanations do. That means the information environment itself is skewed toward the scariest possible answer, independent of actual statistical likelihood.
The escalation pattern researchers found
The original research on cyberchondria found that people often start a search with a mild concern and end it more anxious than when they began, having clicked through a sequence of increasingly alarming results. The searching itself, not new information about the actual symptom, was what drove the escalation.