Mold - The enemy within.

We have one windowless bathroom on the first floor that sees lots of foot traffic from our children. When it became apparent that mold was becoming an issue, we did a couple of things:

we installed a fan to expel moisture and taught the children to use it each time they entered the bathroom
we removed the wall-to-wall carpeting and replaced it with one sheet of linoleum, which stands up to frequent washing with a bleach and water combination
we use that same bleach and water combination regularly on the toilet, including the base of the toilet, and the baseboards.
The bathroom door is left open, except when in use.

The air-tight house that keeps cold air out also serves to keep household moisture in. The showers our family takes every day, along with the moisture created in cooking in the kitchen, running the dishwasher and washing machine, add up to a sizeable amount of moisture created every day in the average home. This can lead to the condition of excess moisture (more than ordinarily expected) which can create decay. Decay is the setting for mold.
Mold is a known cause of allergies. According to the Mayo clinic, these are some signs of allergies to look for:

Irritated eyes
Runny nose
Nasal congestion
Scratchy throat
Coughing
Rashes
Fever
Nausea
Headaches
Dizziness
Confusion
Fatigue

Mold has also been known to trigger attacks in asthmatics. So, it is nothing to take lightly.

Molds sounds so awful that you might think that it'd be easy to spot. This is not necessarily so. First, look for any indications of excess moisture in your home. Some signs of excess moisture in a home may include a musty odor or the feeling of too much humidity. Is there excessive condensation on windows or walls or buckling walls or ceilings? Look for sweating pipes in the kitchen, bathroom, or basement. If any of these conditions occur, look more closely. Mold and mildew may appear on surfaces as a discoloration, which may be white, orange, green, brown or black.

You should also regularly measure the humidity in your home. Invest in a hygrometer, which will let you know when your home's relative humidity (RH) levels are above or below the ideal. For more information on humidity and how it affects your home and your health, visit this URL: http://www.cmhc-schl.gc.ca/en/burema/gesein/abhose/abhose_ce01.cfm

The best prevention of mold is multi-pronged:

Put vents to the outdoors in all bathrooms and kitchens. If you have a heat recovery ventilation system, you may not need to add an additional vent to these rooms.
Put a dehumidifier in your basement.
Clean all mold areas regularly.

If you've already got mold and mildew problems, here are the recommendations of the Canada Mortgage and Housing Corporation (CMHC).

Clean-up methods

Mould and mildew on surfaces can be cleaned up with a solution of one part chlorine bleach in four parts of water.

When applying:

ventilate well
use gloves
let stand 10 - 15 minutes
rinse well
keep surfaces dry

Mould and mildew-stained fabrics should be removed and discarded.