A dangerous germ that has been spreading around the country causes more life-threatening infections than public health authorities had thought and is killing more people in the United States each year than the AIDS virus, federal health officials reported yesterday.

The microbe, a strain of a once innocuous staph bacterium that has become invulnerable to first-line antibiotics, is responsible for more than 94,000 serious infections and nearly 19,000 deaths each year, the Centers for Disease Control and Prevention calculated.