Answer the following question given this paragraph: 

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.


Q: 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?

 Choices:

- superfluous diseases

- nucleus diseases

- cancerous diseases

- viral diseases

A:
viral diseases
