Competition and mutualism can be understood without much attention to the sizes of the species involved. But predation is quite different. Think of a producer, the prey, and a consumer, the predator. When the consumer is extremely large and the producer very small, as with a whale and krill, the relationship is called filter feeding. There the predator kills the prey. When the producer and consumer are roughly matched in size, they are called predators and prey. When the producer is much smaller than the consumer, but still independently mobile, the relationship is called parasitism. And when the consumer is much smaller still, and has difficulty moving around on its own, the relationship is called infection and disease. Symptoms of disease are part of the ecology of disease. In parasitism and disease, the consumer often does not kill the producer.