A vaccine that was developed a hundred years ago to fight the tuberculosis scourge in Europe is now being tested against the coronavirus by scientists eager to find a quick way to protect health care workers, among others.

The Bacillus Calmette-Guerin vaccine is still widely used in the developing world, where scientists have found that it does more than prevent TB. The vaccine prevents infant deaths from a variety of causes, and sharply reduces the incidence of respiratory infections.

The vaccine seems to “train” the immune system to recognize and respond to a variety of infections, including viruses, bacteria and parasites, experts say. There is little evidence yet that the vaccine will blunt infection with the coronavirus, but a series of clinical trials may answer the question in just months.

On Monday, scientists in Melbourne, Australia, started administering the B.C.G. vaccine or a placebo to thousands of physicians, nurses, respiratory therapists and other health care workers — the first of several randomized controlled trials intended to test the vaccine’s effectiveness against the coronavirus.