Vibrio shiloi, the causative agent of bleaching of the coral Oculina patagonica in the Mediterranean Sea, is present in all bleached O. patagonica corals in the summer (25-30°C), but can be not detected in the coral during the winter (16-20°C). Furthermore, the pathogen can not survive in O. patagonica at temperatures below 20°C. Using fluorescence in situ hybridization (FISH) with a V. shilo-specific oligonucleotide probe, we found that the marine fireworm Hermodice caranculata is a winter reservoir for V. shiloi. Worms taken directly from the sea during the winter contained ∼108 V. shiloi per worm by FISH analysis. However, colony-forming units (cfu) revealed only 4.1-18.3 × 104 V. shiloi per worm, indicating that ∼99.9% of them were in the viable-but-not-culturable (VBNC) state. When worms were infected with V. shiloi, most of the bacteria adhered to the worm within 24 h and then penetrated into epidermal cells. By 48 h, less than 10-4 of the intact V. shiloi in the worm gave rise to colonies, suggesting that they differentiated inside the worm into the VBNC state. When worms infected with V. shiloi were placed in aquaria containing O. patagonica, all of the corals showed small patches of bleached tissue in 7-10 days and total bleaching in 17 days. This is the first report of a reservoir and vector for a coral disease.