In an infected person, HIV is found in any body fluid or substance which contains lymphocytes (T4-cell and company). Substances containing lymphocytes include: blood, semen, vaginal and cervical secretions, mother's milk, saliva, tears, urine, and feces.
The presence of HIV within a substance does not necessarily mean the substance is capable of transmitting HIV infection. All of these substances are capable, in theory, of transmitting disease; but in reality, the most dangerous substances seem to be blood, semen, and cervical and vaginal secretions, and perhaps feces. Despite a lot of looking, no one has been able to find a clear cut case of saliva causing transmission, although kissing theoretically could. See "Kissing."
The concentration of HIV in these substances is very important when it comes to infectivity. (Concentration is "number of viruses per unit of volume") If a substance contains a high concentration, that is, a lot of viruses, then it is more likely HIV can be transmitted by the substance. Below a certain concentration of viruses, the substance can not effectively transmit HIV infection.
The importance of concentration is illustrated by the situation with sperm and pregnancy. If a male's semen contains fewer than 20 million sperm cells per milliliter (cubic centimeter), than it is unlikely that the male will be able to impregnate a female. Similarly, if the concentration of a virus is too low in sputum or any other substance, then it is unlikely to transmit infection.
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