Sounds serious… what is it, and where does it come from?
Children are the larval form of the bipedal organism Homo sapiens sapiens (sometimes referred to as “human” or simply “man”), and are as prevalent as, if not more than, the mature form, although their dependency on adult humans limits their expansion to areas already colonized by mature organisms. Children often, though not always, display less coordination, intelligence, and wisdom than their mature counterparts, especially at younger ages, although this becomes less true as the child continues to mature.

Infection (called “conception”) most often occurs during sexual contact, although mature humans (usually those without children of their own) sometimes take a child not directly descended from them as their own. Other methods may exist, of course, but those should be taken on a case-by-case basis. The two given here are the most common.

When infection occurs, it is almost exclusively between a male human and a female human, with the female human becoming the host. Due to lack of proper interfacing equipment, infection occurring between a male and a male or a female and a female is both improbable and impractical. Other forms of disease can, however, be passed between partners at this time, and proper precautions should always be taken (see below).

I think I caught a child. What’s going to happen to me?
The gestation period for a human host carrying a child is 266 days, or about nine months. During this time, the child will increase in size from about 0.14 mm to an average of 17 to 22 inches, with an average weight of six to nine pounds. As this happens, it will become increasingly difficult for the female to perform physical activity. The female host may also experience a wide variety of symptoms, including, but not limited to:

At the end of the nine months, the child will exit the host in a painful process called “labor” and “birth.” The child is not yet autonomous, but through psychophysiological changes in the host, she will usually continue to nurture and care for it until it is able to survive on its own.

I don’t want that to happen again.. ever. How do I prevent it?
The only surefire way to prevent yourself from becoming infected with children or another disease of that nature is to abstain from sexual activity. Barring that, you can use contraceptives to reduce your risk. Note that, while various forms of contraceptives may prevent conception, only a condom that stays on the entire time you are engaged in sexual activity can prevent other forms of sexually transmitted diseases, and even this is not completely certain.

Sources:
http://web.ukonline.co.uk/webwise/spinneret/human2/pregn.htm
http://wy.essortment.com/bodychangesdur_mli.htm