In medical, the Polymerase Chain Reaction (PCR) is a widely used method in molecular biology to quickly make many copies (millions or billions!) of a specific DNA sample (and, in some cases, RNA), which allows scientists to place a very small sample of DNA and amplify it to a sufficiently large amount to study in detail. The key to understanding the PCR is to know that each human being, plant, animal, bacterium, parasite or virus contains genetic material, such as DNA/RNA sequences, unique to its species and to each member of that species. The polymerase chain reaction (PCR) test forms the basis of several tests that help doctors diagnose and treat patients. For example, PCR tests can identify and detect pathogenic organisms in patients, especially those that are difficult to cultivate (for example, HIV and certain fungi and other viruses).