And slowly creeping into your immune system.
It can be a silent killer.
Mold killed a 13-year-old Peoria girl not once, but twice.
"The outside is fixed, but this (ceiling) is what we get on the inside," explains Saprina Brown.
It should've been a simple repair.
A leaky roof.
"The tenants before us had complained about the rain coming through, she was going to send somebody out to fix it, but never did."
After months of water coming into her home...
"To go to sleep and then get a phone call, nine in the morning, saying that your child's dead, the day after her birthday, is just unexplainable," says Brown. "I don't wish that upon nobody."
Mold growing inside gave Brown's 13-year-old daughter, Sheila, an asthma attack so severe, Brown says it killed her daughter, twice.
"I'm thinking we're doing everything right, not knowing it's our home," she explains.
Sheila had just turned 13.
She was staying at her aunt's house for her birthday slumber party.
"I had woke up from my sleep. It was hard for me to breathe. My stomach was hurting," explains Sheila.
The mold inside her home was slowly attacking her body.
"Sheila is gasping for breath," explains her aunt, Sparkle Norman. "Grabbing me around my shoulders. She's trying to say she can't breath, but she couldn't get it out. She collapsed outside and when she collapsed, my mind went blank."
"They told me my baby was dead," says Brown.
Sheila's heart stopped for a minute before paramedics brought her back.
On the way to Children's Hospital of Illinois, her heart stopped again, this time for nine minutes.
"Unexplainable, really," Brown explains. "I didn't know which way to go."
"There was a tube down my throat, says Sheila."I had to learn to walk again, I was on life support and they had me sleep for two weeks. They gave me medicine to paralyze me."
Sheila was in the hospital for nearly a month.
She had to re-learn how to feed herself, how to walk.
Now, she's in therapy, hoping to return to sixth grade, full time, soon.
She says she needs to get back, she has some big goals to work on.
"I'm gonna be a lawyer," says Sheila.
Meanwhile, this family is still facing a rough road.
Because of the mold, there was no way they could go back home.
After weeks of setbacks, they're moving to new home, a new start.
"I never had a doubt in mind, because after she came back to me, I knew God had a plan for Sheila. It was a blessing."
And for Sheila, a second chance.
"I know whatever she puts her mind to do, she'll do," says Brown.
So, how can you prevent mold affecting your family and your home?
Mold experts tell us to make sure the relative humidity in your home isn't more than 50 percent.
You can measure this with a weather gauge.
Check gutters and downspouts to make sure the water is draining away from the house.
Routinely run dehumidifier in the basement, if you have one.
Mold can grow and spread rapidly, so it's important to take care of it, at first sight.
"Once you see evidence of visible mold growth is when you want to contact a professional," says JD Lindsey of Menold Construction and Restoration. "Because at that point, it can be dangerous to your health, if you're not in proper personal protective equipment. At that point, we really recommend contacting a professional."
There's also a lot of warning signs parents can pay attention to when it comes to asthma and their children, as well as protecting against allergies in their homes.
Click here for our special interview with Pediatric Allergy and Pulmonary Specialist, Dr. Kenneth Arnett, of OSF St. Francis Medical Group.