The Red Sea, wedged between Africa and Arabia, stretches from the Suez Canal to the Bab-el-Mandeb Strait. Connecting the Mediterranean Sea with the Indian Ocean, it is believed to have been formed by the separation of the African and Asian tectonic plates. Its extreme climate has been attributed to hot water periodically surging up from hydrothermal vents and brine pools located on its seafloor.

The name Red Sea has been attributed to the seasonal blooms of cyanobacteria (Trichodesmium erythraeum), which turn the water a reddish-brown hue. In 1950, Immanuel Velikovsky claimed in his controversial bestseller Worlds in Collision that, in historical times, major planets had collided or nearly collided with Earth. Specifically, Venus, a massive comet ejected from Jupiter, would have passed close to the Earth around 1500 BCE, causing the dramatic events recorded in Exodus such as the parting of the Red Sea. The scientific community has long rejected Velikovsky’s theories, but it agrees that the Red Sea is among the saltiest and hottest seas on the planet because no major freshwater rivers drain into it.

The Red Sea has played an important role in trade, exploration, religion, and medicine, serving as a major route for commerce among Africa, the Middle East, India, and Europe. For more than 4,000 years, Greek and Roman merchants have transported spices, silk, precious stones, and other valuable goods across it. Millions of pilgrims cross it each year to visit Mecca, and nations near it have fought one another, from antiquity right to the conflicts of our times.

The coral reefs of the Red Sea extend along more than 2,000 kilometers of coastline and harbor some 1,200 fish species. Alongside fish, there are over 200 species of soft and hard corals, sea turtles, dugongs, dolphins, sharks, rays, and invertebrates. The Red Sea corals tolerate heat and high salinity, but for the millions of tourists who snorkel, dive, and swim in the Red Sea, encounters with dangerous marine life are a medical concern. Stonefish, lionfish, fire coral, sea urchins, and jellyfish each present different clinical problems and require different forms of therapy. Diving in deep waters may cause decompression sickness, arterial gas embolism, and barotrauma. Healthcare providers practicing in that area need to familiarize themselves with the specific treatments required for each situation and the available facilities.

The Red Sea basin is one of the most thermally extreme environments in the world. Coastal cities such as Jeddah, Djibouti, and Eritrea’s Massawa experience summer temperatures that routinely exceed 40°C, with high humidity, conditions that create a dangerous combination for outdoor workers, pilgrims, and tourists. Heat exhaustion and heatstroke are significant public health concerns during the Hajj pilgrimage. Millions of Muslim pilgrims travel to Mecca and Medina on the Saudi Red Sea coast each year, many of them elderly or with chronic illnesses, and are exposed to extreme heat during outdoor rituals. Heatstroke—core body temperature above 40°C with neurological dysfunction—is life-threatening and requires immediate cooling. Heat illness can be particularly dangerous during the massive human congregation of the Hajj pilgrimage. Visitors to the area may also be at risk of contracting infections. Schistosomiasis is endemic in the Red Sea region, particularly along the Nile Delta and in sub-Saharan Africa. Vibrio vulnificus and Vibrio parahaemolyticus, bacteria present in warm marine environments, can cause severe wound infections from exposure of open skin to seawater, as well as gastrointestinal illness from consuming raw or undercooked seafood. A distinct clinical syndrome, ciguatera fish poisoning, can be caused by eating large fish, such as barracuda, grouper, and snapper, that have been contaminated with toxins produced by dinoflagellates and bioaccumulated up the food chain by smaller fish. Ciguatera causes a distinctive neurological syndrome, including paradoxical, drastic changes in body temperature.

The Red Sea faces threats from climate change, excessive development along its coast, pollution, overfishing, and the ecological damage wrought by millions of tourists. Rising sea temperatures and ocean acidification pose existential threats to its coral reefs, even if Red Sea corals are more resilient than average. The Red Sea is far more than a shipping lane or a holiday destination. It is a geological wonder still in the process of becoming an ocean, an ecological jewel of extraordinary diversity and resilience, and a medical landscape of profound complexity. It challenges divers and swimmers, threatens the unwary with venom and heat, and harbors emerging infections, but it also offers the scientific community a trove of organisms whose biochemistry may someday yield life-saving medicines. It is essential to protect the people living along its shores and the remarkable natural system that has shaped their civilizations for thousands of years.