They come from an egg laid by an adult female bedbug when that egg hatches. Female bedbugs lay up to 12 eggs every day. Wherever the bedbug egg hatches a nymph bedbug is born there. The nymph can detect the breath of anybody within about 50 metres or more who is asleep. The nymph will instinctively crawl towards there. When it finds the bed where the breath is coming from it will climb up and into it to try to have a meal off the person sleeping by sipping his or her blood. Once it has had its meal it needs to have four more blood meals on subsequent nights before it becomes an adult bedbug, so it will find a hiding place in that bed or else as close as it can to where that person is asleep so it can easily return on subsequent nights to suck the blood of whoever is sleeping in that bed again and again on future nights. Once it has become an adult bedbug, if it is a female it can then lay its own eggs and so start a bedbug family so many more bedbug nymphs may be born in or near that bed, and hence the bed becomes infested with bedbugs.