Normally blood is sterile. Microorganisms can be found in the blood for two reasons: transportation or multiplication.

Microorganisms can enter the body and be transported by the blood to the tissues they infect. For example, the poliomyelitis virus enters through the gastrointestinal tract and is carried to nervous tissue. After microorganisms are established at a site of infection, some can be picked up by the blood. These microorganisms can spread to other locations in the body, multiply within the blood, or be eliminated by the body's immune system.

Septicemia (sep'ti-se'me-ah) or blood poisoning is the multiplication of microorganisms in the blood. Often septicemia results from the introduction of microorganisms by a medical procedure such as the insertion of an intravenous tube into a blood vessel. The release of toxins by bacteria can cause septic shock, which is a decrease in blood pressure that can result in death.

There are a few diseases in which microorganisms actually multiply within blood cells. Malaria (ma-la're-ah) is caused by a protozoan that is introduced into the blood by the bite of the Anopheles mosquito. Part of the development of the protozoan occurs inside erythrocytes. The symptoms of chills and fever in malaria are produced by toxins released when the protozoan causes the erythrocytes to rupture. Infectious mononucleosis (mon'o-nu'kle-o'sis) is caused by a virus that infects the salivary glands and lymphocytes. The lymphocytes are altered by the virus, and the immune system attacks and destroys the lymphocytes. The immune system response is believed to produce the symptoms of fever, sore throat, and swollen lymph nodes. The acquired immune deficiency syndrome (AIDS) virus also infects lymphocytes and suppresses the immune system.

The presence of microorganisms in blood is a concern when transfusions are made, because it is possible to infect the blood recipient. Blood is routinely tested in an effort to eliminate this risk, especially for AIDS and hepatitis. Hepatitis (hep'a-ti'tis) is an infection of the liver caused by several different kinds of viruses. After recovering, hepatitis victims can become carriers. Although they show no signs of the disease, they release the virus into their blood or bile. To prevent infection of others, anyone who has had hepatitis is asked not to donate blood products.