Why is it harder to determine exposure to airborne transmitted disease than exposure to bloodborne transmitted disease?

The reason for this is that we can trace bloodbourne diseases back to a direct source, we an collect samples, and identify an approximate, if not exact, number of people infected. The problem with airbourne disease is that air doesn't stop moving. We can't measure the exact number of people who inhaled the specific portion of infected air, who then breathe it in another location, and may transmit more of the pathogens in that area.

Put simply, bloodbourne pathogens are more confined, spread less quickly, and are easier to track than airbourne ones.