In modern parlance, the word pest,  as in the term pest control  has come to refer to a myriad of small creatures that invade the house, and bring with them filth and disease. Typical pests include mice, roaches, termites, bedbugs, fleas and other insects, wild birds, wild animals, spiders and snakes. But when we go back into  time, we find the word had a different twist. the word pest didn't refer to the animals we call pests today, but to the deadly filth and disease which they carry.

Lets examine the facts. Checking in Etymon's online etymology dictionary, we find that the term pest  appears first and foremost in various imprecations, such as "a pest (or a plague) upon you." Going back further in time to 1539, we find that the word "peste" in Middle French, had the same meaning as its antecedent "pestis" in Latin, and meant deadly contagious disease.

Etymological derivations of other pest  related words indicate that "pestis" also meant "plague." The modern word pestilence, derived from the 13th century Old French word pestilence, and from the Latin pestilential meaning plague, noun of action from pestilentem (nom. pestilens). In this derivation, Entymon notes that pestilens derives from pestis, which can also mean plague. 

A mouse,  in and of itself,  is not a deadly contagious disease. However, a mouse carries Leptospirosis, a dangerous microbe, and it can also carry dangerous or even deadly viruses. Likewise, a pigeon,  is a bothersome creature, especially when it uses our porches and sidewalks as a bathroom, but it isn't contagious. However, pigeons can carry and transmit histoplasmosis, and other dangerous or deadly respiratory conditions. The plague, the deadly scourge of the Middle Ages, was transmitted by rats  and fleas. While rats and fleas both bite, they are not, in and of themselves causes of contagious diseases. However, they do transmit disease causing viruses and bacteria. such as the organism that caused the  plague.s that ravaged Europe during the Middle Ages. Bats, another frightful household pest are not in and of themselves deadly. However, their bite transmits the virus which causes rabies,  a progressive invariably fatal condition.  Not only their bite, but in some cases their well aimed spit can transmit this incurable virally transmitted illness.

This historical background provides perspective that enables us to understand the true meaning of the phrase pest control ,which was first seen in use about 200 years ago. In a clasical sense,  pest control means controlling pests, (deadly viruses and microbes which cause disease), by removing the animals and other creatures which carry these illness causing organisms from the domestic environment . But in modern parlance, the original associations is often lost, and people relate the term pest to the creatures and animals that spread filth and disease, and not the creatures that cause disease. 

It must be noted that  when taken in this light, the term pest control  has come to have new connotations. Classically, for example pest control and mice control were not synonymous. It would be  more proper to say that mice removal is a form of pest control because we are removing disease carrying rodents from our home. However, we could control mice by putting them in a cage, as is done in a laboratory, and this might not remove the pests they carry from our environment. Termites do not carry disease, and yet "termite control" is considered to be a form of "pest control" in modern parlance. These usages lead further credence to the belief  that the words pests, like many other words, has taken on an enlarged meaning.

In the classical sense, vaccination, isolation and testing might classically be considered forms of pest control, becasue they help to protect us from disease causing viruses and microbes, but in general  pest control  has specifically come to refer to the removal of small creatures between the size of mites to the size of large animals, which carry filth and disease into the home.