Sickness and disease does not just randomly happen. It occurs for specific reasons.
You are not perfectly healthy until you get sick, nor does perfect health return once sickness has run its course, although people tend to perceive their health this way.
Remember your cells. Only after massive numbers of cells malfunction or die do you begin to notice symptoms of sickness and disease, like a headache, fever or a cold.
In other words, you are already sick before you "get sick."
Because sickness and disease is a process, even if it is something as simple as a cold.