The cold is the most common human ailment. Most adult Americans suffer from one to four colds per year, but children ages one to five—who are the most susceptible—typically may contract as many as eight. Colds are spread by respiratory droplets or by contaminated hands or objects. Although the incidence of colds is higher in winter, exposure to chilling or dampness is considered to be of little significance.
Any one of up to 200 viruses (such as the rhinoviruses, coronaviruses, or respiratory syncytial virus [RSV]) can cause colds, to which it seems almost no one is immune. Infection with a viral strain confers only temporary immunity to that strain. Colds in infants and young children caused by RSV can progress to pneumonia and other complications, especially in those under a year old who were born prematurely or have chronic lung disease; RSV causes an estimated 4,500 deaths yearly in these groups in the United States.
There is no treatment for the common cold other than that aimed at relieving symptoms and keeping the body well-rested, -fed, and -hydrated. Because of the growing problem of drug resistance, doctors are being discouraged from prescribing antibiotics (which do not affect viruses) for colds unless secondary bacterial infection makes them necessary. There is no convincing evidence that vitamin C megadoses can prevent the common cold.
Researchers have reported reduction or prevention of cold symptoms in human tests of an experimental drug against rhinoviruses, which cause nearly half of all colds. The drug acts by imitating a molecule in the body called ICAM-1, to which the rhinovirus attaches to produce colds. As rhinoviruses attach to the decoy molecules instead, the likelihood or severity of infection is decreased.
Cold-blooded organisms (called poikilotherms - "of varying temperature") maintain their body temperatures in ways different from mammals and birds. The term is now outdated in scientific contexts. Cold-blooded creatures were, initially, presumed to be incapable of maintaining their body temperatures at all. Cold-blooded animals are now called ectotherms, a term which signifies that their heat (therm) comes from outside (ecto) of them; the term cold-blooded is misleading.
Advances in the study of how creatures maintain their internal temperatures (termed: Thermophysiology) have shown that many of the earlier notions of what the terms "warm-blooded" and "cold-blooded" mean, were far from accurate (see below: Definitions). Today scientists realize that body temperature types are not a simple matter of black and white. Most creatures fit more in line with a graded spectrum from one extreme (cold-blooded) to another (warm-blooded).
Cold-bloodedness generally refers to three separate areas of thermoregulation.
Few creatures actually fit all three of the above criteria. Most animals use a combination of these three aspects of thermophysiology, along with their counterparts (endothermy, homeothermy & tachymetabolism) to create a broad spectrum of body temperature types. Most of the time, creatures that use any one of the previously defined aspects are usually pigeon-holed into the term cold-blooded.
Physiologists also coined the term heterothermy for creatures that exhibit a unique case of poikilothermy.
Many homeothermic, or warm-blooded, animals also make use of these techniques at times. For example, all animals are at risk of hypothermia on cold days, and most homeothermic animals can shiver to get warmer.
Poikilotherms often have more complex metabolisms than homeotherms (homopathics). For an important chemical reaction, poikilotherms may have four to ten enzyme systems that operate at different temperatures. As a result, poikilotherms often have larger, more complex genomes than homeotherms in the same ecological niche. Frogs are a notable example of this effect.
Because their metabolism is so variable, poikilothermic animals do not easily support complex, high-energy organ systems such as brains or wings. Some of the most complex adaptations known involve poikilotherms with such organ systems. One example is the swimming muscles of Tuna, which are warmed by a heat exchanger. In general, poikilothermic animals do not use their metabolisms to heat or cool themselves. For the same body weight poikilotherms need ⅓ to 1/10 of the energy of homeotherms. They therefore eat only ⅓ to 1/10 of the food needed by homeothermic animals.
Some larger poikilotherms, by virtue of their substantial volume to surface area ratio, are able to maintain relatively high body temperatures and high metabolic rates. This phenomenon, known as gigantothermy (inertial homeothermy), has been observed in sea turtles and great white sharks, and was most likely present in many dinosaurs and ancient sea reptiles (such as ichthyosaurs and plesiosaurs). For example, some species of sea turtles are homeothermic some of the time. They float on the surface of the ocean to absorb heat and then, after submerging again, stay homeothermic for periods of time because of their sheer size. During long periods of time underwater their body temperature may decrease, depending on the temperature of the surrounding water. Their body temperature may also decrease when they float on the surface of the ocean at night, depending on the surrounding temperature.
However, large dinosaurs were probably not poikilotherms, but homeotherms (homeothermic all the time) due to the overwhelming mass of their bodies.
This energy difference also means that a given niche of a given ecology can support three to ten times the number of poikilothermic animals as homeothermic animals. However, in a given niche, homeotherms often drive poikilothermic competitors to extinction because homeotherms can gather food for a greater fraction of each day.
Poikilotherms succeed in some niches, such as islands, or distinct bioregions (such as the small bioregions of the Amazon basin). These often do not have enough food to support a viable breeding population of homeothermic animals. In these niches, poikilotherms such as large lizards, crabs and frogs supplant homeotherms such as birds and mammals.