A pesticide is any substance used to kill or repel insects, weeds, bacteria, mold, or viruses. They are used in homes and on lawns, in agricultural fields on crops, and in parks, golf courses, and buildings. Exposures to pesticides may cause a wide variety of both acute (short-term) and chronic (long-term) health effects, depending upon the type of pesticide, and the amount of exposure, and how long the exposure lasted. Chronic exposure to some pesticides may increase the risk for certain types of conditions (including cancers and birth defects), cause damage to the immune system and to the genome, and affect development. Children may be especially vulnerable to pesticide exposure because they are still developing.