The AIDS epidemic is sweeping out of control among crack cocaine smokers in the country's poorest neighborhoods but is still barely perceptible across much of the nation, new data show.

Several reports presented this week at an AIDS meeting sponsored by the American Society for Microbiology demonstrate the vast differences in how the epidemic touches Americans' lives.One study found that in New York City and Miami, 4 percent of crack cocaine users are becoming infected annually with HIV, the AIDS virus.

This infection rate is "among the highest ever reported in the United States," said Dr. Brian R. Edlin of the federal Centers for Disease Control and Prevention in Atlanta.

But outside hot spots of AIDS infection, mostly in big cities and the rural South, the situation could hardly be more dif-fer-ent.

A new analysis, released Tuesday, showed that just six-hundredths of 1 percent of people applying for insurance are infected.

The study, conducted by the CDC's Dr. J. Todd Weber, was based on a review of blood samples sent to one lab that does AIDS testing for hundreds of insurance companies nationwide. The samples are provided by people applying for health, life and disability insurance.

Of nearly 1.5 million samples tested in 1991 and 1992, just 886 turned out to be infected with HIV. This low infection rates is similar to those seen among first-time blood donors and military recruits.

Among the tiny minority who were infected, HIV was most common in Washington, D.C.; Puerto Rico, Florida and New York state.

This study suggests AIDS has not made deep inroads into mainstream American life. However, it does not provide a true cross-section of AIDS infection since some people who suspect they are infected may refuse to give blood for testing.

Another CDC study involved women giving birth. It found that 1.6 of every 1,000 were infected. Over time, the infection rate in these young women has gradually gone down in the Northeast, slowly increased in the South and stayed low and stable in the rest of the country.

HIV infections also appear to be steady or decreasing in two of the main groups at risk, gay men and needle drug users.