Infected

When was the exact moment you caught that cold or other infectious disease, when the bacterium or virus (etc.) that would eventually multiply throughout your body first entered your system? What action brought it into your body, and through which orifice? Whom did you catch it from?

The problem is, I cannot foresee science and technology ever being able to reliably answer these questions. It seems one would have to monitor and track every object inside and outside your body at microscopic resolution.