lethal

An injury or substance that can cause death is lethal.

You might hear people talking about poisonous plants being lethal to pets. A lethal dose of poison means the amount needed to kill someone. Some viruses are so contagious and deadly, they're called lethal. If something is lethal to a certain type of animal, it's fatal for those animals when ingested or exposed to them in some way. The word "lethal" often describes things that cause death quickly or easily.