If it's been a while since you last cleaned your carpet, then you might not be too worried. After all, if it doesn't look filthy, then it's probably fine, right? Unfortunately, there are many health problems that might be lurking beneath the surface of your carpet. For instance:
Mold can be a pretty serious health condition, especially if anyone in your house has a compromised immune or respiratory system. Spores can easily travel around the house, embedding themselves in your lungs and causing serious discomfort, if not health problems. On top of that, mold can actually be a lot harder to detect than you might imagine.
Mold tends to grow in damp, dark, warm areas, so its location might not be immediately obvious. It might have originated in a leak in your pipes in the floor, walls, or ceiling, but mold can grow fairly rapidly. In particular, if you do have carpets in areas of your home that are near any sources of water, such as sinks, toilets, or showers, then you should always be vigilant about the growth of mold. If you have a type of carpet that has long fibers, then you want to be doubly careful, since there could be a layer of mold that is concealed by the thick carpet.
A little less terrifying are dust mites, which are small organisms that thrive in dirty carpets. They can eat the organic material that naturally builds up in carpets, breeding and multiplying. The real problem comes when they die, since their corpses are actually fairly dangerous to humans when inhaled. They certainly aren't lethal, but they can trigger allergies and can cause quite a bit of irritation to the eyes, nose, and throat.
Finally, you want to be vigilant about athlete's foot, which is caused by fungal infection. Dirty carpets can be a breeding ground for these fungi, and if you happen to walk around your carpet while your feet have cuts or some other avenue for foreign material to enter your body, then you can easily catch athlete's foot as well. This can become even more problematic if you live with someone who already has a case of athlete's foot. Every time such a person walks around on your carpet with an infected foot, they increase the risk that everyone else will catch their disease as well, and that is the last thing you want.
For assistance in removing the dangers, call a professional carpet cleaner.