Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Viral diseases can be difficult to treat. They live inside the cells of their host, so it is hard to destroy them without killing host cells. Antibiotics also have no effect on viruses. Antiviral drugs are available, but only for a limited number of viruses. Answer: viral diseases
What kind of diseases can be difficult to treat because they live inside the host's cells, making it hard to destroy them without killing host cells?