1. Fatal or causing death in more than 50 percent, but fewer than 100 percent of cases: The doctors were cautious regarding the new disease that was spreading around, considering it a semilethal illness, not consistently fatal but still highly dangerous.
2. A mutation that produces more than 50 percent of mortality but not everyone: A change that was caused in the genetic make up of the monkeys at the zoo was considered semilethal; therefore, the researchers believed only about half of the species would survive.