If they are poorly maintained, heat / cooling pumps will become incubators for bacteria which can make your entire family sick. Individuals suffering from asthma are especially at risk.
Most homeowners don’t know this, but it is recommended that heat/cooling pumps be cleaned once a year.
Most manufacturers’ manual does talk about the importance of cleaning and maintaining heat/cooling pumps, but rarely does one mention the risk of mildew if cleaning is not done properly. This goes without mentioning that most people don’t even bother reading the manufacturers’ manual at all.