What percentage of townhomes have any sort of shared ventilation?

Let's say, for example, we experienced a major flu pandemic similar to
that which occurred in 1918, and we were living in a townhome.

Flu is an airborne virus. Therefore, if you share any ventilation with
your neighbors and their townhouse became contaminated, it is possible
you could actually catch the virus from contaminated air from the
shared ventilation system in the townhouse complex, if any shared
ventilation exists.

This actually happened during the SARS crisis, where many occupants of
a single apartment tower caught SARS from the shared ventilation
system. My question is, how often is it that townhomes share
ventilation of any kind?

Obviously I am aware that if they share a hallway with other
townhomes, that is the same as shared ventilation. Please assume we're
talking about townhomes that have their own entrance and no shared
hallway/shared entrance.

The person who can answer this question should actually know how most
townhomes are constructed and the degree to which ventilation is
shared in townhome buildings.