Essex was one of the first to link animal and human retroviruses to immunosuppressive disease, to suspect that a retrovirus was the cause of AIDS, and to determine that HIV could be transmitted through blood and blood products to hemophiliacs and recipients of blood transfusions.
Essex played a key role in establishing the connection between animal and human retroviruses and immunosuppressive disease, hypothesizing that a retrovirus was the likely cause of AIDS, and demonstrating that HIV could be transmitted through blood and blood products to individuals with hemophilia and those receiving blood transfusions.