Select the word that means “so gradual as to not become clearly visible for a long time”.
Eventually, the physical manifestations of the disease became apparent, but the insidious, virulent illness did not present physical symptoms for an extended period of time.

Insidious means “so gradual as to not become clearly visible for a long time.”
The medication was insidious – it made Karen feel better at first but slowly created dangerous side effects after months of use.
Manifestation means “indication or sign of an ailment.” It is another word for symptom.
A physical manifestation of the disease was uncontrollable coughing.
Apparent means “visible.”
It became apparent that the patient had no intentions to follow the nurse’s regimen of diet and exercise.
Virulent means “extremely harmful or severe.”
Ebola is a virulent disease. It can take over a host with infection extremely quickly.