It's not uncommon for an illness to run through an entire household, as each member passes their contaminants on to the others. But it's a bigger issue if many members of your household keep getting mysteriously ill on a regular basis. When that happens, it's time to take action to find out the causes of recurring illness.
Approach a Physician
One thing that you can do is check with a physician to determine potential causes of the illness. You could write down the symptoms that you're experiencing, and interview your household members to see which symptoms are common among you. This will give a doctor some great starting ground for potential solutions to try.
Check for Environmental Contaminants
When several people in one area are getting sick repeatedly, it's a good idea to check their shared environment to see if there are any environmental factors. One example is carbon monoxide poisoning; you may be experiencing a repeated exposure to the chemical that is causing mysterious illness. Since HVAC systems are a common source of carbon monoxide in the home, have your contractor check for leaks and install carbon monoxide detectors in your system.
Clean Household Surfaces
It may be that some source of bacteria or viruses is lingering on the surfaces in your home. For instance, if someone was ill and they used food preparation surfaces and equipment, and then those surfaces weren't cleaned properly, you may be getting repeatedly exposed to a virus. This is a good opportunity to go through and thoroughly sanitize home surfaces and make sure everyone is on the same page about food preparation safety.
Check Your Water Supply
Another potential source of recurring illness in a household is contamination of your water supply. This is especially likely if you have your own water supply, such as a well, but it can also happen if there are sources of contamination between your municipal supply and your home.
If you suspect that water contamination is a problem, you could try getting your water samples sent to a lab that can test for viruses and bacteria as well as other common contaminants. It's also wise to consult with a water treatment specialist so that they can help you set up an appropriate filtration system. Or, you could skip the water supply altogether and get a bottled water supplier for your household. Figuring out the sources of recurring illness in your home can be a process of trial and error, but these are some great places to start.