The number of Americans dying from cancer has continued to fall since peaking in 1991, averting 1.5 million deaths that otherwise would have stemmed from one of the most feared diseases in the world.

The American Cancer Society, in its annual statistical report, estimated this week that 1.7 million Americans will be diagnosed with malignancies and 589,430 will die from them in 2015, roughly 1,600 people a day. Cancer already is the leading cause of death among Americans ages 40 to 79, and is expected to overtake heart disease as the nation’s top killer in the next several years, said chief executive John Seffrin.

Just four types account for half of all cancers in the U.S.: prostate, breast, lung and colorectal tumors.

Progress has been made in reducing the incidence of prostate, lung and colon cancers, while breast cancer rates have leveled off in recent years, according to Cancer Statistics 2015.

The drops in the past two decades have been enormous. Death rates from breast cancer are down by one-third from their peak, while prostate and colon cancer deaths have been cut in half.

Officials attribute the decline in death rates to a dramatic reduction in smoking rates, along with improvements in preventing, detecting and treating tumors. Heart disease death rates are falling even faster than the rates for cancer, also spurred by the decline in smoking and preventive medicines.

“The continuing drops we’re seeing in cancer mortality are reason to celebrate, but not to stop,” Seffrin said in a statement. “We can still lessen cancer’s deadly impact by making sure as many Americans as possible have access to the best tools to prevent, detect, and treat cancer.”