What is an undetectable viris?

Answer 1

Usually this term is used when somebody is known to be infected with a virus, such as HIV or Hepatitis C. After treatment, the virus is reduced to such a low level that it cannot be detected by our most sensitive tests. It doesn't mean you've been cured though, since the virus could still be present at very low levels though and return when the treatment stops. But the treatment is working well.