Its mathematical expression does not propagate in the ordinary sense of the word, though it may refer to virtual or nominal velocities with magnitudes greater than that of light. For example, wave packets are mathematical objects that have group velocity and phase velocity. The energy of a wave packet travels at the group velocity (under normal circumstances); since energy has causal efficacy, the group velocity cannot be faster than the speed of light. The phase of a wave packet travels at the phase velocity; since phase is not causal, the phase velocity of a wave packet can be faster than light.Causal notions are important in general relativity to the extent that the existence of an arrow of time demands that the universes semi-Riemannian manifold be orientable, so that "future" and "past" are globally definable quantities. Engineering
A causal system is a system with output and internal states that depends only on the current and previous input values. A system that has some dependence on input values from the future (in addition to possible past or current input values) is termed an acausal system, and a system that depends solely on future input values is an anticausal system. Acausal filters, for example, can only exist as postprocessing filters, because these filters can extract future values from a memory buffer or a file. Biology, medicine and epidemiology
Austin Bradford Hill built upon the work of Hume and Popper and suggested in his paper "The Environment and Disease: Association or Causation?"
Cachexia () is a complex syndrome associated with an underlying illness, causing ongoing muscle loss that is not entirely reversed with nutritional supplementation. A range of diseases can cause cachexia, most commonly cancer, congestive heart failure, chronic obstructive pulmonary disease, chronic kidney disease, and AIDS. Systemic inflammation from these conditions can cause detrimental changes to metabolism and body composition. In contrast to weight loss from inadequate caloric intake, cachexia causes mostly muscle loss instead of fat loss. Diagnosis of cachexia can be difficult due to the lack of well-established diagnostic criteria. Cachexia can improve with treatment of the underlying illness but other treatment approaches have limited benefit. Cachexia is associated with increased mortality and poor quality of life. The term is from Greek κακός kakos, "bad", and ἕξις hexis, "condition". Causes
Cachexia can be caused by diverse medical conditions, but is most often associated with end-stage cancer, known as cancer cachexia. About 50% of all cancer patients develop cachexia. Those with upper gastrointestinal and pancreatic cancers have the highest frequency of developing a cachexic symptom. Prevalence of cachexia rises in more advanced stages and is estimated to affect 80% of terminal cancer patients.Congestive heart failure, AIDS, chronic obstructive pulmonary disease, and chronic kidney disease are other conditions that often cause cachexia. Cachexia can also be the result of advanced stages of cystic fibrosis, multiple sclerosis, motor neuron disease, Parkinsons disease, dementia, tuberculosis, multiple system atrophy, mercury poisoning, Crohns disease, trypanosomiasis, rheumatoid arthritis, and celiac disease as well as other systemic diseases.