For example, if A denotes the event "The person is a smoker," B denotes the event "The person now has or will have cancer at some time in the future" and C denotes the event "The person now has or will have emphysema some time in the future," then the following three relationships hold: P{B|A} ≥ P{B}, P{C|A} ≥ P{C} and P{B|C} ≥ P{B}. The last relationship states that knowing that the person has emphysema increases the likelihood that he will have cancer. The reason for this is that having the information that the person has emphysema increases the likelihood that the person is a smoker, thus indirectly increasing the likelihood that the person will have cancer. However, we would not want to conclude that having emphysema causes cancer. Thus, we need additional conditions such as temporal relationship of A to B and a rational explanation as to the mechanism of action. It is hard to quantify this last requirement and thus different authors prefer somewhat different definitions. Causal calculus
When experimental interventions are infeasible or illegal, the derivation of a cause-and-effect relationship from observational studies must rest on some qualitative theoretical assumptions, for example, that symptoms do not cause diseases, usually expressed in the form of missing arrows in causal graphs such as Bayesian networks or path diagrams.
Purpura () is a condition of  red or purple discolored spots on the skin that do not blanch on applying pressure. The spots are caused by bleeding underneath the skin secondary to platelet disorders, vascular disorders, coagulation disorders, or other causes. They measure 3–10 mm, whereas petechiae measure less than 3 mm, and ecchymoses greater than 1 cm.Purpura is common with typhus and can be present with meningitis caused by meningococci or septicaemia. In particular, meningococcus (Neisseria meningitidis), a Gram-negative diplococcus organism, releases endotoxin when it lyses. Endotoxin activates the Hageman factor (clotting factor XII), which causes disseminated intravascular coagulation (DIC). The DIC is what appears as a rash on the affected individual. Classification
Purpura are a common and nonspecific medical sign; however, the underlying mechanism commonly involves one of:

Platelet disorders (thrombocytopenic purpura)
Primary thrombocytopenic purpura
Secondary thrombocytopenic purpura
Post-transfusion purpura
Vascular disorders (nonthrombocytopenic purpura)
Microvascular injury, as seen in senile (old age) purpura, when blood vessels are more easily damaged
Hypertensive states
Deficient vascular support
Vasculitis, as in the case of Henoch–Schönlein purpura
Coagulation disorders
Disseminated intravascular coagulation (DIC)
Scurvy (vitamin C deficiency) – defect in collagen synthesis due to lack of hydroxylation of procollagen results in weakened capillary walls and cells
Meningococcemia
Clumping fibrillary protein deposits caused by Amyloidosis
Cocaine use with concomitant use of the one-time chemotherapy drug and now veterinary deworming agent levamisole can cause purpura of the ears, face, trunk, or extremities, sometimes needing reconstructive surgery. Levamisole is purportedly a common cutting agent. Decomposition of blood vessels including purpura is a symptom of acute radiation poisoning in excess of 2 Grays of radiation exposure.