In the 1970s, scientists noticed that soft-shell clams along the east coast of North America were dying from a strange type of cancer. Their blood, which was typically clear, would fill with so many cells that it would turn milky. The rogue cells clogged and infiltrated the clams’ organs, often killing them.

This cancer—this clam leukaemia—seemed to be transmissible. If you took the blood of infected clams and injected it into healthy individuals, some of those recipients would develop the disease. For years, scientists suspected that a virus was involved

The clam leukaemia is a contagious cancer—an immortal line of selfish shellfish cells that originated in a single individual and somehow gained the ability to survive and multiply in