Bloodstream infections (BSIs) include bacteremias when the infections are bacterial, and fungemias when the infections are fungal, are infections present in the blood. Blood is normally a sterile environment, so that the detection of microbes in the blood (most commonly accomplished by blood cultures) is always abnormal. A bloodstream infection is different from sepsis, which is the host response to bacteria. Bacteria can enter the bloodstream as a severe complication of infections (like pneumonia or meningitis), during surgery (especially when involving mucous membranes such as the gastrointestinal tract), or due to catheters and other foreign bodies entering the arteries or veins (including during intravenous drug abuse).  Transient bacteremia can result after dental procedures or brushing of teeth. Bacteremia can have several important health consequences. The immune response to the bacteria can cause sepsis and septic shock, which has a high mortality rate. Bacteria can also spread via the blood to other parts of the body (which is called hematogenous spread), causing infections away from the original site of infection, such as endocarditis or osteomyelitis. Treatment for bacteremia is with antibiotics, and prevention with antibiotic prophylaxis can be given in high risk situations. Gram positive bacteria are an increasingly important cause of bacteremia. Staphylococcus, streptococcus, and enterococcus species are the most important and most common species of gram-positive bacteria that can enter the bloodstream. These bacteria are normally found on the skin or in the gastrointestinal tract.

Staphylococcus aureus is the most common cause of healthcare-associated bacteremia in North and South America and is also an important cause of community-acquired bacteremia. Skin ulceration or wounds, respiratory tract infections, and IV drug use are the most important causes of community-acquired staph aureus bacteremia. In healthcare settings, intravenous catheters, urinary tract catheters, and surgical procedures are the most common causes of staph aureus bacteremia. There are many different types of streptococcal species that can cause bacteremia. Group A streptococcus (GAS) typically causes bacteremia from skin and soft tissue infections.  Group B streptococcus is an important cause of bacteremia in neonates, often immediately following birth. Viridans streptococci species are normal bacterial flora of the mouth. Viridans strep can cause temporary bacteremia after eating, toothbrushing, or flossing. More severe bacteremia can occur following dental procedures or in patients receiving chemotherapy. Finally, streptococcus bovis is a common cause of bacteremia in patients with colon cancer.

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