What difficulties arise when trying to rigorously apply Koch's postulates to show that HIV causes AIDS?

It is unethical to infect healthy person with hiv virus and virus cannot be grown in a pure culture.

Explanation:

Koch's postulates says that microbes are the cause of any disease. These microbes can be isolated from the host and can be grown in a pure culture. This pure culture when injected in a healthy host it will cause the same disease and then from the infected person the same microbe can be isolated again.

When we apply Koch's postulates in HIV case several difficulties will arise like as virus require host cell to reproduce therefore it is not possible to reproduce a virus in a pure culture and as it is unethical to infect a healthy individual to show the cause of disease, therefore, we can follow the Koch postulate here.