I recently moved into my house and I keep getting a mildew like smell more so when the bathtub is drained but strange thing is, it seems to me that the smell is not coming out of the drain but out of the ac duct that lies about 6 inches from the tub. I hope someone can help, my concern is the health of my family. Thank you!

It may not be plumbing problem at all. You probably have mildew inside your (heating ?) duct. Open the duct register and inspect the interior. If there is any visible sediment, clean it / vacuum it out. Buy sanitizing spray in local supermarket and spray it everyday until it is gone. You just have to hope the mildew didn't spread deep into the duct. If positive, than you have to replace section of the duct.