No, visitors are not covered by a specific CERN personal accident or health insurance simply because they are visiting CERN.

CERN has comprehensive safety rules and procedures in place for visits. Visitors must follow the safety instructions given by CERN staff and guides, respect the applicable restrictions and remain with their guide during guided visits. CERN also has an onsite Fire and Rescue Service, which is available in case of an incident or emergency. 

CERN also maintains general insurance coverage, including third-party liability insurance, in connection with the Organization's activities.
Visitors should therefore ensure that they have appropriate personal health/accident and, where applicable, liability insurance coverage for their visit.