A pesticide may be a chemical substance or biological agent (such as a virus or bacteria) used against pests including insects, plant pathogens, weeds, mollusks, birds, mammals, fish, nematodes, and microbes. These organisms may compete with humans for food, invade lawns and gardens, destroy wood in houses, spread disease, or just be a general nuisance in our everyday lives. Pesticides are usually, but not always, poisonous to humans.
There are 5 major types of pesticides you will encounter: insecticides (insect killers), herbidicides (weed killers), fungicides (fungus killers), nematocides (roundworm killers), and rodenticides (rat and mouse killers). Be aware that although the 5 are listed here, there are many others (e.g. algicides, miticides).
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