Years ago I lived in a very cheap cottage that had no heat. About the same time I worked in an office in which my desk was next to a closet full of mold. Although I had a large air purifier running constantly, I became ill. I've had an allergy to mold ever since, and I also get bronchitis every winter. One doctor even diagnosed asthma as a result of the exposure to mold.
Although I'm normally scrupulous about checking potential apartments for mold, after weeks of searching for a pet-friendly rental, I'd been exhausted and desperate. The apartment seemed clean, and I didn't pull up the window blinds. It was a serious mistake.
When I mentioned the mold to the property manager, she said she'd seen it but thought it was just dirt -- which doesn't explain why it was ignored by cleaners and painters. The company's handyman recommended sandblasting, as did mold remediation experts. I was told I'd be let out of my lease.
I can't imagine not opening up windows for ventilation, but it's obvious the previous tenant, who lived in the apartment four years, didn't. I can't imagine not cleaning the windows (they can't be cleaned now without risking more spreading of the mold spores), or ignoring the mold -- but the previous tenant, the property management company, the cleaners, and the painters all did. My advice: Ventilate, open windows, run fans, clean condensation off windows. Keep an eye out for mold -- if you see it, do something about it right away. And always check the windows when looking at potential rentals.