Cockroaches are nasty creatures. They leave a trail of filth behind them and shed their exoskeletons as they grow (seeing these sheddings in your home is often the first sign of a cockroach infestation.) They need water more than food, but can go weeks without either. For every one you see there can be as many as 100 others you can't see. They cause allergic reactions and increase the risk of asthma to children.
The first instinct that many people have when they learn they have a cockroach infestation is to run out and buy every kind of poison they can find to kill them all. They use bug-bombs, roach traps, roach baits, roach sprays, and roach gels to carpet bomb their home. The problem is, this stuff isn't good for people either and roaches breed so fast they can become immune to poisons. In fact, what usually happens is they avoid the poisoned areas until you stop the treatments and then they move back in.