Changes can happen quickly in smaller, simpler organisms. For example, many bacteria that cause disease can no longer be killed with some antibiotic medicines. These medicines have only been in use since the 1940s, and at first, they worked extremely well. The bacteria have evolved so that they are less affected by antibiotics. The drugs killed off all the bacteria except a few which had some resistance. These few resistant bacteria reproduced, and their offspring had the same drug resistance.