This past summer doctors treated two Americans infected with Ebola virus with an experimental drug created by Mapp Biopharmaceutical. Both patients lived, although experts are not certain whether the drug contributed to their survival. Named ZMapp, it is a mixture of different antibodies that bind to the virus—and is made by tobacco plants.

Plants do not have antibodies of their own, but they nonetheless have the cellular machinery to make these infection-fighting proteins. Researchers first recognized such potential in 1989 and went on to hijack a tobacco plant's biology to synthesize human antibodies. Since then, several biotech companies have been developing plantibodies that could treat diseases, such as Ebola and rabies.

Plantibody production is straightforward: scientists insert the gene for an antibody into a disarmed virus, which is taken up by a plant's leaves. Using the new DNA, the plant builds the human proteins. Scientists extract them about a week later. The process takes a little over a month—a faster and cheaper means of manufacturing than using hamster ovary cells, which is the standard. Growing the plants is inexpensive, says Julian Ma, an immunologist at St. George's, University of London. “It's basically just soil and water you're paying for.”

Despite its ease, plantibody production is not widespread. Most large pharmaceutical companies are reluctant to make the switch because they have invested so much money in ovary cells, Ma says. Until plantibody drugs go through regulatory processes, smaller biotech companies most likely will be the ones producing them.

Plantibodies in development include those designed to target HIV, herpes, cancer and rabies. ZMapp itself is nearly ready to enter clinical trials: a recent study of Ebola-infected monkeys demonstrated its effectiveness. Experts estimate that plantibodies will not go on the market for at least five years, but that projection may change. In September the U.S. Department of Health and Human Services announced that it would like to accelerate ZMapp tests in an 18-month push.