Hepatitis C is disease that affects thousands. It can go undetected and undiagnosed for decades. In fact, the disease was first called non-hepatitis A or B. The medical community first started diagnosis Hepatitis C in 1989.
Many veterans were infected, unbeknownst to them, in the 1950s, 60s, and 70s with Hepatitis C while they were in service. But they were never diagnosed with the illness till many years later.
The VA has recognized that many veterans were exposed to Hep C and it has developed a list of risk factors to include: Organ transplants before 1992, blood transfusions before 1992, hemodialysis, exposure to blood, intravenous drug use, high sexual activity, and tattooing. Even though the VA recognizes these exposures the VA still denies veterans service connection for Hep C because there is no nexus between service and the current disease.
Remember that to obtain service connection you have to show something happened in service, that there is a current disability and that there is a nexus (i.e. link) between the two. I describe the nexus as a bridge. The closer you are from your time of discharge to filing a claim (say you were discharged this year and filed a claim the next month) the bridge between service and the current disability is short and easy to show. On the other hand, if you were discharged in the 1970s and are now claiming a disability based on something that happened decades ago the bridge is a long one and harder to prove.
It is the long bridge scenario that I have seen veterans have to deal with in Hep C cases. Especially veterans who served in Vietnam, almost twenty years before the medical community could even diagnose this disease. Since the disease can be dormant for years and not show itself for a long time this becomes an obstacle for veterans to overcome, because the VA will say that since the symptoms did not start for years after service it must not be related. This reasoning is against the weight of the medical literature on Hep C. Do not let VA deny a valid Hep C claim for this reason.