Asymptomatic

adj. without symptoms

In medicine, a disease is asymptomatic while the patient does not experience any noticeable symptoms. Asymptomatic diseases may not be discovered until the patient undergoes medical tests (X-rays or other investigations). Some diseases remain asymptomatic for a remarkably long time, including some forms of cancer. If a patient is asymptomatic, precautionary steps must be taken.
See more at Wikipedia.org...

A·symp·to·mat·ic adj. Exhibiting or producing no symptoms. Asymptomatic Persons who carry a disease and are usually capable of transmitting the [more]Asymptomatic - Community and Resources

Adjective
1. having no symptoms of illness or disease
(synonym) symptomless
(similar) well

adjective
which does not show any symptoms of disease