How can we prove that HIV causes AIDS?

Koch's Postulates

In the nineteenth century, the German scientist Robert Koch developed a set of four "postulates" to guide people trying to prove that a germ causes a disease. Scientists agree that if HIV satisfies all of these conditions with regard to AIDS then it must be the cause of AIDS:¹⁵

• Koch 1: The germ must be found in every person with the disease
• Koch 2: The germ must be isolated from someone who has the disease and grown in pure culture
• Koch 3: The germ must cause the disease when introduced into a healthy person
• Koch 4: The germ must be re-isolated from the infected person