There was once a time when houses could be neglected in many ways and not suffer the way they will today if neglected. This became less true as soon as we started to insulate our homes to save energy.
The interiors of older homes, for all intents and purpose, communicated very well with the exterior.
They were drafty.
This draftiness allowed for a sort of built-in means of changing the air on a regular basis. In fact the air changes were more or less continuous. The key to saving energy was to figure out a way of stopping these air changes.
So we started, insulating, and taping, and insulating, and caulking, and insulating, and vapor sealing---and then insulating some more. The result was that we started having houses that were very successful at stopping air movement and we no longer had to spend 1/2 the year cutting wood to heat them.
But along with this saving of ax handles, energy and money came an unanticipated cost.
Poor indoor air quality.
Poor indoor air quality manifested itself in the trapping of all sorts of nasty particulates inside the home. Chemicals out-gassing from interior components and furnishings, dust from pets and human occupants, and the growth of mold due to elevated moisture levels and poor circulation, were just some of the things became trapped in the home. There was also the byproducts of combustion related to fossil fuels and solid fuels burned in the home. Many other sources of indoor air pollution contributed to this soup of poor indoor air: air fresheners, incense, candles, detergents, hair spray, paints etc.
So as a result, after figuring out that we could indeed stop air movement altogether, we figured out that it was a very bad idea and that we better figure out a way to have "controlled" air movement.
In modern homes we provide a way to mechanically ventilate the home---to provide air changes.
Every time we turn on an exhaust fan, we are removing air from the home and "theoretically" we are pulling fresh air into the home somewhere else. These fans can, and should, be placed on timers to maintain ventilation according to predetermined times. Usually leakage around doors and windows will suffice as a place for fresh air to be pulled into the home. 
Modern doors and windows however, are so well weatherstripped and constructed, that very little air leaks into the home at these points and when we turn the fans on they literally do not pull as much air from the home as they should be and the home experiences negative pressure. Air can then be pulled into the home through the other exhaust fan locations (the dampers in these fans will still let some air by) or down the b-vent chimneys or the fireplace chimneys. I think everyone can appreciate that using the furnace b-vent as an air intake is not a good idea. It will also result in air being pulled under the baseboards or around crawl space hatches, leaving that characteristic "black ghosting" that everyone worries is mold---but is in fact just dirt being filtered by the carpet as air comes and goes from the building.
Because of this we want to provide actual locations where the air can come-and-go from the home whenever the indoor environment is placed under positive or negative pressure. Sometimes this is done by little vents in the windows or walls or by automatic dampers that are part of an air intake into the forced air heating system. Most structures deal fairly well with small pressure differentials but when they become great enough, moisture vapor can be pulled or pushed to places we don't want moisture to be.
With two or three bathroom exhaust fans, the kitchen range hood, the gas furnace/water heater, and the dryer all running at the same time we can create quite a bit of negative pressure and that air that is being exhausted has to come from somewhere.
The vast majority of homes in the United States have inadequate means of balancing the air that is being exhausted.
It is not rocket science as to why so many homes suffer from air quality issues right along with their occupants.
Perhaps the best way of all to manage this air exchange is to install an HRV or Heat Recovery Ventilator. These are great because as they exhaust air from the home the outgoing air stream passes over the incoming air stream so that the warm air leaving the home can heat up the cold air coming in---and visa versa in Summer. Of course the air is filtered in the process as well.
In the State of Washington we are lucky enough to have an Energy Code that is very specific about the installation of these air exchange systems. Even if you live in a state without an energy code (or perhaps especially if you don't), I consider it prudent to not only ask the following question but to understand its answer: "When I turn on an exhaust fan in my home, how is fresh air getting in to replace the air that is leaving?" If you cannot answer that question or did not even know that the question should be asked, you may not adequately understand how to take care of your house and your home may be at risk of poor indoor air quality.
While there are other important things to know, it is the control of the air changes in the home that is perhaps the single most important thing that every owner (or tenant) of a modern home needs to understand---and yet, based on my experience as a home inspector, very few do.
A home inspector with some training in building science (and shouldn't they all?) can help you access how your home is functioning and/or how it should be functioning.
P.S. Today, Reuben Saltzman and I were on the same page in terms of this topic---please see his post about insulating your rim joists. In that post there is a link about Combustion & Makeup Air that is very important no matter what state you live in. It provides very good infomration that covers what I talk about in this post but in much greater detail.
Charles Buell, Seattle Home Inspector
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