How did the “agent” jump easily from person to person?

Richard Preston's The Hot Zone is a vivid account of the study of viral hemorrhagic fevers. It begins with outbreaks of the Ebola and Marburg viruses in central Africa and then moves to controlled experiments at USAMRIID, the US Army Medical Research Institute of Infectious Diseases, where military scientists infect rhesus monkeys with these viruses to learn how the diseases progress and whether they can be stopped.
The viruses cause severe vomiting and diarrhea. This is then followed by a weakening of blood vessels throughout the body, which leads to internal and external bleeding in the infected person, who dies of massive blood-loss. The viruses are referred to as "infectious agents" by the scientists in their experiments, and the experiments begin with the commonsense idea that transmission occurs through physical contact with bodily fluids—people or animals exposed to the blood, vomit, or feces of infected persons or animals have a very high risk of contracting the disease.
The breakthrough that occurs at the Reston labs is the discovery that monkeys in separate rooms, who have no contact with one another, are somehow getting infected, which means the viruses must spread by air as well as physical contact. The virus that affects the monkeys is a variant of the Ebola virus, dubbed the "Reston virus," which, fortunately, does not seem to cross the species barrier and affect humans.