Their immune systems were already strong enough when they got hep. C. They didn't do anything special. The virus never became active. Mine was that way until I took 1 1/2 bottles of kava kava. Then that aggravated my liver and the virus became active. You can read about this 15% in most any book on Hepatitis C. Like the Hepatitis C Handbook and many others.
Viruses never leave the body, they are either active or dormant. You are never cured. It is a technicality. We can never donate blood or organs even after it goes dormant.