Pesticides are substances used to kill insects, plants and other organisms that negatively impact crop yield. They can range from hazardous, artificially-isolated chemicals, such as many organochlorides, to relatively innocuous plant-based preparations, like neem oil. Pesticides can have unintended consequences such as killing off beneficial, predatory insects.
Most of the pesticides in our food, by far, are natural pesticides produced by the plants. This leaves open the question of whether the artificial chemicals are worse for us. After all, not all substances are the same, and some (such as DDT) linger in the environment for far longer. It's also true that something is harmful given to lab rats in large quantities, yet not significantly harmful in small quantities - or even beneficial, since there has been research suggesting that toxins in small doses actually benefit an organism by making it react to the mild stress.[verification needed]
Many natural chemical compounds are also toxic or carcinogenic in large quantities, but we consume them in small quantities. Everything has a toxic dose - even water, salt or any nutrient.
There is a common perception that "the poisons are killing us." So why are we living longer than ever? If there is a negative effect from these traces of chemicals, the effect is much smaller than positive changes in modern times (e.g. better medicines and medical treatments).
Note that these arguments are not saying that "pesticides are good for you" - using them inappropriately, without following directions, has the potential to be very harmful. But when used properly, they appear to not be significantly harmful, and may not be harmful at all. Worrying about them may do us more harm than the chemicals themselves.