WASHINGTON — A Frankenstein version of the “Spanish flu” virus, assembled from parts in the laboratory, has shed new light on how the microbe killed tens of millions of people worldwide in 1918 and 1919.

Experiments in monkeys show that the 1918 virus came with the pre-packaged capacity to limit the immune system’s ability to fight back in the first few days after infection. As the virus grows unchecked, the body attacks it with increasing quantities of highly toxic substances, which over time do as much harm to the host as to the invader. The result is often lethal damage to the lungs, where most influenza virus growth occurs.

The research provides further evidence that the 1918 virus had traits not found in other flu viruses but that it was the body’s frantic effort to fight it that ultimately killed many of its victims.

“We know that the virus itself is different, and we know that the host response is different,” said Yoshihiro Kawaoka, a virologist at the University of Wisconsin, who headed the international research team.

The new experiment was done in a Canadian biosafety level 4 lab, where researchers work in the equivalent of space suits. Mr. Kawaoka’s team does not have permission to experiment with the virus in America.

Because of its extreme hazard, only a single group of American researchers, at the Centers for Disease Control and Prevention, in Atlanta, is currently allowed to use whole, “living” reconstructed versions of the pathogen that killed at least 50 million people nearly a century ago.

The new research, being published in the journal Nature, is possible because in 2005 American researchers successfully completed the laborious work of copying the 1918 virus’s genetic blueprint, or genome, using fragments of tissue from three victims of the pandemic. That permits scientists to synthesize the microbe using a process called “reverse genetics,” which Mr. Kawaoka developed in the 1990s.