Before the AIDS epidemic was identified, Haseltine already had the expertise and the facilities to study human retroviruses.

Although many retroviruses were known to infect animals, only two human retroviruses were known; the AIDS virus became the third.

Cell-to-cell transmission through virological synapse in human retroviruses is also inhibited by tetherin.

Subsequently after, other oncogenic human retroviruses, such as Epstein-Barr virus were discovered.

This finding was not replicated, and no evidence of infection with human retroviruses was found.

Experiments on mice and other mammals have demonstrated the relevance of retroviruses in leukemia, and human retroviruses have also been identified.

Today Dr. Gallo said he agreed the theory did not explain the origin and spread of all human retroviruses.

Notable human retroviruses include HIV-1 and HIV-2, the cause of the disease AIDS.

For example, some putative new human retroviruses have turned out to be animal viruses or other contaminants.

"It is a very interesting result that needs confirmation, but raises the possibility that we are only looking at the tip of the iceberg in terms of human retroviruses."