First lady Laura Bush said Sunday that the U.S. government is right to eavesdrop on Americans with suspected ties to terrorists and that President Bush worries that revelations about the domestic spying program will cripple efforts to foil terrorists.

"I think the American people expect the United States government and the president to do what they can to make sure there's not an attack by foreign terrorists," Mrs. Bush said just before landing here to begin a four-day stay in West Africa.

"I think he was worried that it would undermine our efforts by alerting terrorists to what our efforts are," she said.

Mrs. Bush planned to travel on Monday to Liberia to attend the inauguration of President-elect Ellen Johnson-Sirleaf, the first woman president on the continent. She also is highlighting U.S.-backed education and HIV-AIDS programs in Ghana and Nigeria.

In a 12-minute exchange with reporters on the plane, Mrs. Bush rebuffed criticism that too much of U.S. assistance for battling AIDS in Africa is focused on abstinence programs. She said abstinence, the use of condoms and being faithful to one's sexual partner are all important in curbing the spread of disease.

"I really have always been a little bit irritated by criticism of abstinence because abstinence is absolutely, 100 percent effective in fighting a sexually transmittable disease," she said. "When girls are not empowered, girls are vulnerable, and their chances of being able to negotiate their sexual life with their partners and to make their partner chose a condom are very low."

She said that in Africa, where one in three people have AIDS, a sexually transmitted disease, it's important to talk about abstinence.

"In many countries where girls feel obligated to comply with the wishes of men, girls need to know that abstinence is a choice. It's really very, very important to have all three," she said.

Mrs. Bush also quelled speculation that Vice President Dick Cheney, who has suffered heart attacks in the past but not as vice president, would step down before the end of Bush's term. Cheney recently used a cane because of a foot ailment.

"I think Vice President Cheney's in great health and he's doing great," Mrs. Bush said. "Yes, I'm sure the president would not like for him to step down, obviously. The vice president has been an excellent vice president. He's solid as a rock."