Mammals have been admired and harvested or domesticated for food, clothing and other resources since early times in human history.

Yet, rodents feel the wrath of humans whenever your housemate says your room looks like a "rat's nest" or calls you a "dirty rat."

However, our dislike for all rodents is largely unwarranted; instead, we can pretty much blame our negativism toward rodents on just three commensal species of rodents: house mice, black rats and Norway rats.

Commensal refers to sharing the table with humans and relates to the close association that these species have with humans.

Fear and dislike of these commensal rodents began many centuries ago in the Old World. Mouse outbreaks were common and dreaded phenomena in Asia and Europe that historically resulted in large numbers of mice wandering the countryside eating almost anything edible.

Outbreaks of rat populations struck fear in humans because these rodents were hosts to fleas and lice, which carried infectious, fatal human diseases. From about 3,500 years ago to the year 1720, 195 plagues were recorded in Europe and Asia -- these plagues were later attributed to diseases transmitted by high populations of black and Norway rats.

In fact, one-third of the human population in Europe died from these plagues during the Middle Ages.

One of the diseases was the bubonic plague, which was last recorded in the United States (in Los Angeles) in 1924. It is caused by a bacterium that is transmitted by flea bites or fecal contamination associated with rats; it is fatal to humans in 30 to 75 percent of all cases.

In short, the three commensal rodents seem to be troublemakers everywhere they go, which today is virtually worldwide. In Pennsylvania, house mice are common on farms and even occasionally occur as noncommensals in managed forest stands.

Likewise, Norway and black rats are found in a variety of situations in close association with humans or in more natural habitats.

Who invited these commensals to the New World?

House mice and black rats arrived in the New World on ships with early colonists and explorers in the 17th and early 18th centuries.

Norway rats made their way on ships to the New World a little later, arriving sometime around the American Revolution.

Once the aggressive Norway rats became established, they easily displaced black rats from most of their range.

As an aside, for you history buffs -- what commensal mouse was a Roman emperor? Julius Cheeser.

Besides being vectors of human diseases, what other problems are attributed to these rodents? Norway and black rats inflict bites at an estimated rate of one in every 100,000 humans. Rat bites usually afflict young children and sickly adults, causing physical and emotional harm. On rare occasions, rat bites can be fatal.

These commensals can have indirect effects on people by damaging property and food. Economic loss attributed to the two rat species (Norway and black combined) is at least $10 per year, per rat (1970 U.S. dollars) or about half a billion to $1 billion each year in the United States.

Commensal rats and mice gnaw on insulation in human dwellings, causing about 25 percent of the fires of undetermined origin in homes and buildings. These commensal rodents feed on crops and stored grain intended for consumption by humans or farm animals. From a conservation perspective, commensal rodents have caused ecological disasters when they set foot in any new land. This has been particularly true on islands, where endemic fauna such as ground-nesting birds are very vulnerable to predation by introduced populations of black and Norway rats. Birds and other fauna on islands typically evolved in the absence of predators and are thereby defenseless against aggressive predators like rats.

One positive indirect effect of commensal rodents has been their value as lab animals in medical and behavioral research. Studies conducted on white mice and rats, which are domesticated versions of house mice and Norway rats, respectively, have provided beneficial information for an understanding and treatment of many physical and mental diseases in humans such as alcoholism, obesity and stress.