If you think there is a mold problem you could look for any sources of moisture or leaks and remediate those. The CDC has some guidance on where to start https://www.cdc.gov/mold-health/about/index.html
If moisture is a problem in areas like the bathroom you can install an automatic humidity controlled switch for the fan. It will regulate humidity automatically after showers.
If you feel like you must do something more, a HEPA air purifier would remove mold spores from the air. A carbon filter would remove odors and other compounds. A building-level solution is a heat recovery ventilator which brings in fresh air but captures most of the heat from the exhaust air.
I know you want to find a source of the health problem and identify something you can fix, but mold is probably not it. Mold is a popular topic in complementary and alternative medicine as an explanation for everything. If you ask the internet if any health condition could be caused by mold, someone will show up and tell you yes. I need to caution you that there are forums and subreddits where this is taken to extremes and anyone who disagrees has their comments deleted, creating an illusion of consensus. Some of these forums will tell you that the only solution is to move to a dry climate and only sleep outside. There are mold testing companies where your results will always come back positive followed by a recommendation that you buy their expensive consulting services. Please take some caution and a healthy dose of skepticism before getting involved with this subject.
