High temperatures typically cause corals to become more susceptible to disease. When coral reefs experience thermal stress, they can undergo a process known as coral bleaching, where they expel the zooxanthellae (the symbiotic algae) that live in their tissues. This not only leads to a loss of color but also weakens the corals, making them more vulnerable to diseases and less able to recover from stress. Therefore, the correct response is:
corals become more susceptible to disease.