My guess is it's in your home -- not one place, but several. The first place I'd look is behind your shower curtain. Then maybe I'd look over your shoulder as you open your refrigerator. If I was an appliance repairman, I'll bet you've got it in your washing machine, too. It's everywhere. It's mold.
Mold comes in all sorts of varieties, but the most common one you might have is black mold. The spores that fuel mold growth are everywhere in your home. They're outside in your yard and on your deck. They're in your attic. Forget about trying to get rid of them. It's nearly impossible.
Fortunately you can minimize and just about eliminate mold growth in your home. Mold is similar to fire. To have a fire, you just need three things: fuel, heat and oxygen. For mold to grow, it just needs three things: mold spores, food, water. It's nearly impossible for you to eliminate the first two things in your home, but you can often control water.
DEGREE OF DIFFICULTY: ONE HAMMER OUT OF FIVE
STEP ONE. Shower mold is the toughest of all problems to solve. Your shower and tub area is paradise for mold because you and your family flood the walls and floor with water not once, but probably multiple times each day. You need to get everyone who uses the shower to just do a few things to stop the black menace.
STEP TWO. Start by cleaning the bathroom. Do everything possible to get rid of existing mold. Not only will you thoroughly clean the shower, but you'll also need to wash down all surfaces in the room to get rid of invisible mold spores that are everywhere.
STEP THREE. Deep mold growth in tile grout may require multiple cleaning attempts. Saturate paper towels with the bleach solution and plaster these towels to the grout lines. Keep the paper towels wet with the bleach for hours. You'll see improvement each time you do this. Don't forget to take off the shower drain strainer and clean all the mold from the underside as well as the sides of the drain pipe above the water seal of the trap. The sides of the pipe will be coated with gooey black mold. Do the same in your vanity sink.
STEP FOUR. Once the bathroom is free of all, or most of, the mold, now it's time to control the water. After each shower, the walls of the shower must be squeegeed. Do the floor, too. You want to get as much water off the walls and down the drain as possible.
STEP FIVE. If you have a shower curtain, shake as much water from it as possible. Consider using an old towel to dry off the curtain and the shower walls. If you have an exhaust fan, run it.
STEP SIX. Leave the shower curtain open as well as glass shower doors. You need the shower area to dry as fast as possible. Consider using a fan you might use in the summer to stay cool. There are quite a few nice ones that are small and can be turned on as you do your grooming in the room. Leave the bathroom door open after you leave it so the humidity in the room equalizes with the rest of the house as rapidly as possible.
STEP SEVEN. To stop mold in your refrigerator, you need to cut off the fuel or food supply. Clean up all the spills of food that happen on the shelves, door and door seals. Make a habit of doing a thorough cleaning of your refrigerator every three months. No excuses.
STEP EIGHT. Washing machines are favorite places for mold to grow. You can stop mold in its tracks here by removing food sources and controlling water. The soaps you use to clean your clothes are food for mold. The dirt from your clothes is also fuel. Some of this dirt and soap get on surfaces inside the machine above the water line from splashing. Front loading machines have this gunk all over the seals and doors. Clean it off each week. Leave the door to the washing machine open after each use.
SUMMARY. To control mold growth anywhere, the key is to get rid of water. On cold winter days, leave closet doors open where you may have mold growing on walls. If the exterior walls get some additional warmth from the room, invisible water vapor may not form on the walls. This fog, just like you see on a mirror, fuels mold growth on cold or cool walls and ceilings.
Use floor or portable fans to evaporate water from surfaces where you can't see the water vapor. Without water, the mold spores just lay there and wait.