Petechiae are caused by minor bleedings from capillaries, tiny blood vessels that lie under the skin and mucous membranes. Although many causes can lead to petechiae, the underlying mechanisms typically involve damage to the capillary walls, causing blood leakage, or a low blood platelet count (i.e., thrombocytopenia), which can lead to spontaneous bleeding in the absence of trauma.
Tiny petechiae appearing on the face, neck, and upper chest are often caused by rupture of capillaries due to prolonged straining during a bowel movement or severe bouts of coughing or vomiting. Petechiae may also occur as a result of traumatic injuries, sunburns, or as a side effect of certain medications, such as antibiotics and anticonvulsants.
In children, infections are a common cause of petechiae. Pathogens associated with petechiae include viruses, like cytomegalovirus (CMV), Epstein-Barr virus (EBV), or influenza; bacteria, including Neisseria meningitidis and group A Streptococcus species; and fungi, such as Candida and Aspergillus species. Other, less common infections that can lead to petechiae include Rocky Mountain spotted fever, which is transmitted by a tick bite, and viral hemorrhagic fevers, such as dengue, Ebola, and yellow fever. Overall, infections can lead to petechiae through several different mechanisms, including damage to the blood vessel walls, toxin release, or uncontrolled activation of the coagulation cascade, which in turn can lead to a low blood platelet count.
Other than certain infections, pathological causes of petechiae include bone marrow failure, an acquired or inherited disorder leading to decreased production of platelets, white blood cells, and red blood cells; and leukemia, a type of blood cancer. Other disorders associated with thrombocytopenia include immune thrombocytopenic purpura (ITP), an autoimmune condition leading to platelet destruction; hemolytic uremic syndrome (HUS), a condition often triggered by infections leading, among other things, to a low platelet count; and disseminated intravascular coagulopathy (DIC), an uncontrolled activation of the coagulation cascade leading to increased platelet consumption and thrombocytopenia. Finally, petechiae may be a sign of systemic conditions, including blood vessel wall inflammation (i.e., vasculitis), connective tissue disorders with increased blood vessel fragility, chronic liver disease, and vitamin C or vitamin K deficiency. 
