While the function of interleukin-4 in the immune system is well understood, its application as a means to suppress immune response and negate vaccine effectiveness was brought into sharp focus by the mousepox experiments. Though its effect on the human immune response to similar viral challenges is not yet clear, there is concern that the technique could be applied to other infectious agents. It could be particularly devastating if IL-4 were engineered into a common virus like influenza that generally kills very few people, but spreads very easily.

Smallpox is caused by the Variola virus, which is spread through person-to-person contact and results in death in up to 30 percent of cases. Those who survive smallpox are often left with permanent scarring or blindness.

Smallpox has been a scourge across recorded history. The last case in the United States was in 1949. With the eradication of smallpox by the World Health Organization in 1979 (the last case being in Somalia), routine vaccination of the public has been halted for a generation. While official stocks of Variola only exist in two locations - the CDC laboratory in Atlanta and a Russian state laboratory in Koltsovo - the use of smallpox as a bioweapon is of considerable concern. President Bush implemented a program in 2002 to vaccinate all health care and emergency workers as well as the military against smallpox, but did not provide the public with a specific reason for the program. The mousepox experiment demonstrates that the simple insertion of the IL-4 gene into the smallpox virus could conceivably render these precautions and the vaccine stockpile useless.