Perhaps it would be more accurate to say someone had no obvious indications of underlying health problems rather than saying that they were healthy. Do we even know in any precise way what healthy means? Plus I believe that fitness is often used as as marker for health but they can be two different things. One can be fit but not truly healthy. People die running marathons. Finally it is a kind of medical arrogance to assume that we know how to measure accurately disease and/or health. We know some indicators of both but the human body is more complex than our current knowledge understands. We can say definitively that some people are sick but we probably miss a lot of people because they appear "healthy" but aren't. We assume more than we know.