Cause
M. leprae and M. lepromatosis
M. leprae and M. lepromatosis are the mycobacteria that cause leprosy. M. lepromatosis is a relatively newly identified mycobacterium isolated from a fatal case of diffuse lepromatous leprosy in 2008. M. lepromatosis is indistinguishable clinically from M. leprae.M. leprae is an intracellular, acid-fast bacterium that is aerobic and rod-shaped. M. leprae is surrounded by the waxy cell envelope coating characteristic of the genus Mycobacterium.Genetically, M. leprae and M. lepromatosis lack the genes that are necessary for independent growth. M. leprae and M. lepromatosis are obligate intracellular pathogens, and can not be grown (cultured) in the laboratory. The inability to culture M. leprae and M. lepromatosis has resulted in a difficulty definitively identifying the bacterial organism under a strict interpretation of Kochs postulates.While the causative organisms have to date been impossible to culture in vitro, it has been possible to grow them in animals such as mice and armadillos.Naturally occurring infection has been reported in nonhuman primates (including the African chimpanzee, the sooty mangabey, and the cynomolgus macaque), armadillos, and red squirrels. Multilocus sequence typing of the armadillo M. leprae strains suggests that they were of human origin for at most a few hundred years. Thus, it is suspected that armadillos first acquired the organism incidentally from early American explorers.
Causes
Central cyanosis
Central cyanosis is often due to a circulatory or ventilatory problem that leads to poor blood oxygenation in the lungs. It develops when arterial oxygen saturation drops below 85% or 75%.Acute cyanosis can be a result of asphyxiation or choking and is one of the definite signs that ventilation is being blocked. Central cyanosis may be due to the following causes:

Central nervous system (impairing normal ventilation):Intracranial hemorrhage
Drug overdose (e.g. heroin)
Generalized tonic–clonic seizure (GTCS)
Respiratory system:Pneumonia
Bronchiolitis
Bronchospasm (e.g. asthma)
Pulmonary hypertension
Pulmonary embolism
Hypoventilation
Chronic obstructive pulmonary disease, or COPD (emphysema)
Cardiovascular system:Congenital heart disease (e.g. Tetralogy of Fallot, right to left shunts in heart or great vessels)
Heart failure
Valvular heart disease
Myocardial infarction
Hemoglobinopathies:Methemoglobinemia 
Sulfhemoglobinemia 
Polycythemia
Congenital cyanosis (HbM Boston) arises from a mutation in the α-codon which results in a change of primary sequence, H → Y. Tyrosine stabilises the Fe(III) form (oxyhaemoglobin) creating a permanent T-state of Hb. Others:
High altitude, cyanosis may develop in ascents to altitudes >2400 m.
Hypothermia
Frostbite
Obstructive sleep apnea

Peripheral cyanosis
Peripheral cyanosis is the blue tint in fingers or extremities, due to an inadequate or obstructed circulation. The blood reaching the extremities is not oxygen-rich and when viewed through the skin a combination of factors can lead to the appearance of a blue color. All factors contributing to central cyanosis can also cause peripheral symptoms to appear but peripheral cyanosis can be observed in the absence of heart or lung failures. Small blood vessels may be restricted and can be treated by increasing the normal oxygenation level of the blood.