Estimates range from the tens of thousands to more than 300,000, according to the CDC.

Molds release spores. They are tiny, reproductive cells. The spores are carried by air, water, or insects. People are actually allergic to the spores. Allergy symptoms include nasal stuffiness and eye irritation. More severe reactions may also occur. Mold allergies can make symptoms worse or cause flare-ups in some people with asthma.

Molds grow better in warm, humid conditions such as the basement or the shower, the CDC says. Other common indoor sites include pipes, damp walls, and houseplants. Damp ceiling tiles, cardboard, and wood also grow mold.

Mold spores are constantly drifting through the air, the CDC says. Once the spores are in your house and drop on places that have excessive moisture, they will grow. Some places might surprise you: dust, paints, wallpaper, drywall, carpet, fabric, and upholstery.

Molds grow outdoors in shady, damp areas. They are also found in antique shops, greenhouses, saunas, flower shops, and summer cottages. On farms, mold is often found in hay, straw, or grain.

It doesn't matter what kind of mold is growing. The most important thing is to get rid of it, the Environmental Protection Agency (EPA) says. Professional testers are expensive. Whether you or someone else does the testing, no standards have been established for acceptable levels of mold.

You can remove mold growth with commercial mold-killing products, soap and water, or with a weak bleach solution (1 cup of bleach for each gallon of water), the CDC says. Be sure to open windows and doors to allow fresh are in if you're using bleach. Bleach is not advised for routine use. After cleaning, dry the area completely.

The main way to control mold growth is to get rid of moisture as much as possible. You can do this by keeping the humidity low in your house. An air conditioner or dehumidifier can help during humid weather. Other tips: Be sure your home has good ventilation, including exhaust fans in the kitchen and bathrooms. Fix leaky plumbing. Use mold-killing products when you clean the bathrooms. Remove or replace any carpets or upholstery that has gotten wet.