An unknown disease or a well-known disease that suddenly becomes harder to control is called
a. a pathogen.
b. an emerging disease.
c. an antibiotic.
d. a viral disease.

The best answer is B.

Infectious diseases whose incidence has increased  in the past two decades or threatens to increase in the near future have been said to be "emerging''. Such diseases include the following:

1. Old  infections emerging as a result of antimicrobial resistance in known agents or break down in public health measures.

2. New infections resulting from change or evolution of existing organisms.

3.  Known infections spreading to new geographical areas or population.

4. Previously unrecognized infections in areas undergoing ecologic transformation.