Tests that directly measure a virus in the blood have contributed a great deal to understanding the biomedical basis of HIV infection. Measurement of “viral load” the concentration of viruses in the bloods a valuable tool for evaluating the effectiveness of therapeutic drugs. Viral load has also been found to influence the individual’s chances of transmitting the virus by sexual and other means.


Thus a therapy that is effective in reducing viral load can help to control the spread of HIV. Similar diseases in animals can be transmitted by injecting brain tissue from an infected animal into the brain of a previously healthy one. However, no virus or bacteria has been found responsible for causing the condition.


In 1997, Stanley Prusiner, a scientist at the University of California Medical School in San Francisco, won the Nobel Prize for his controversial theory that this type of disease is caused by particles called prions, which contain protein but no nucleic acid and thus no traditional genetic material. A flurry of an alarm and a European ban on British beef led the British government in 1996 to order the mass slaughter of all at-risk cattle to prevent the possibility of human exposure to infected beef.