You might think that your carpet looks clean, but is it really? The thing I have learned is that most of the nasty stuff, such as dust mite feces, is so tiny that you'll never actually see it. Just imagine the size of the dust mites themselves. You could fit thousands of them on one fingernail. Now imagine how small their feces must be and now imagine it being all over your carpets.
So, is vacuuming once in a blue moon good enough? Hardly. You have to make sure that you are not just vacuuming every day but that you are vacuuming in the correct manner. Make sure that you are covering every inch of carpet and that you are running the vacuum in many different directions in order to make sure that you are really covering those carpet fibers.
Additionally, it is important to make sure that your carpet is cleaned well with a hot water system at least every six months. You want to make sure that you do not do it anymore than every three months as this could cause damage to the padding under your carpet and the glue that holds your carpet fibers together.
If you do not already do this, you might want to consider calling in a professional carpet cleaning service at least once a year. For those who can afford the extra expense, have them come out twice a year. If you watch for advertised specials, you should be able to get some good deals.