Dr. Indiana Torres of Puebla General Hospital said 22 percent of the 1,000 or so cases of H.I.V. and AIDS that her clinic handles can be traced to migration, mostly to the New York area.

PUEBLA, Mexico — Cres has spent almost half his 32 years working in the United States, in the fields of California and Texas and the factories of Chicago and New York. His wife and three children were with him some of the time. But he was alone for long spells, and it was during one of those periods that he figures he contracted H.I.V.

“I don’t know how or where or when I got it,” said Cres, who spoke on condition that he would be identified only by his nickname. He paused whenever his pregnant wife entered the darkened home, built with his paychecks from America. “I don’t have any idea who it was with. I don’t want to know. I just want to go ahead with my life.”

Migrant workers like him go to the United States with dreams of new prosperity, hoping to bring back dollars. But some are bringing back something else as well, H.I.V. and AIDS, which they are spreading in the rural parts of Mexico least prepared to handle the epidemic.

As sweeping proposals for immigration-law changes founder in the United States, the expanding AIDS crisis among the migrants is largely overlooked on both sides of the border. Particularly in Mexico, AIDS is still shrouded by stigma and denial. In the United States, it is often assumed that immigrants bring diseases into the country, not take them away

But AIDS is spreading quickly in rural Mexican states with the highest migration rates to the United States, researchers say. The greatest risk of contracting AIDS that rural Mexican women face is in having sex with their migrant husbands, a new study found, a problem that is compounded by their husbands’ refusal to use condoms.

Research has shown that migrants have more sexual partners than those who stay at home. For women, life on the road brings risks of rape and sexual abuse. For many migrants, being displaced from their homes and families is a lonely experience, one that prompts them to form new relationships in the United States

Adding to the problem, both Mexico’s northern and southern borders have become magnets for prostitutes and drug dealers, drawn by the flow of migrants north

“Migration leads to conditions and experiences that increase risks,” said George Lemp, an epidemiologist who directs the University of California’s AIDS research program and is studying the spread of the disease among migrants. “Migrants are vulnerable. They are isolated. They are exposed to different sexual practices. They have language barriers to services and there is a lot of depression and loneliness and abuse.”

AIDS has not yet exploded in Mexico and is focused mostly among prostitutes and their clients, and drug users and gay men, experts say. The AIDS rate here is still considerably lower than that in the United States, nearly half as low, according to United Nations statistics published in 2006. The H.I.V. infection rate for people ages 15 to 49 in the United States is about 0.6 percent, compared with 0.3 percent in Mexico, the United Nations says

Yet the high-risk behavior that various surveys have documented among many Mexican migrants worries researchers. “Our concern is it could take off in this population in the future,” said Dr. Lemp, who is leading a joint United States-Mexican study of migrants and AIDS

The first AIDS cases diagnosed in Mexico in 1983 were found among migrants, researchers say. Since then, studies have continued to show that migrants to the United States make up a significant percentage of those contracting the disease.

The percentage of Mexicans with H.I.V. who have lived in the United States fluctuated between 41 percent and 79 percent in the 1980s and early 1990s, studies have shown. But since 1992, Mexico has not reported comprehensive figures

Still, recent studies show the risks that migrants face. A study financed by the California-Mexico AIDS Initiative found that more than a third of the migrants at job-pickup sites in Los Angeles had been offered money by men for sex. About a tenth of the migrants, desperate to earn a living, have agreed, the study found

Many migrant husbands have sex with people more likely to have H.I.V., have limited access to health care and frequently cope with “the social isolation of the migrant experience by seeking comfort in sexual intimacy,” Jennifer S. Hirsch, a professor of public health at Columbia University, wrote in The American Journal of Public Health in June.

She found that unfaithful migrant husbands who were otherwise devoted to their wives were often the highest risk. They were more likely, she said, to seek sex with prostitutes while in the United States and less likely to have long-term relationships with other women

The risks were compounded because the subject of unfaithfulness is frequently taboo within relationships. “Men’s long absences lower the reputational risk of infidelity by ensuring that it occurs far away,” she wrote

At Puebla General Hospital, Dr. Indiana Torres said 22 percent of the 1,000 or so cases of H.I.V. and AIDS that her clinic handles can be traced to migration, mostly to the New York area. A new more spacious clinic is under construction to handle the load

“They think that because it’s the United States, it’s safer,” Dr. Torres said. “It’s their fantasy and it’s not true.”

One of the women in the emergency room at the hospital, a 25-year-old mother who spoke on condition of anonymity, said her husband had infected her after returning from a long stay in Washington State.

She found out that she carried the virus only after giving birth to a girl who was born with H.I.V. and died. An older daughter also contracted the virus from birth. The woman and her husband have since separated.

Doctors say routine screening for H.I.V. is not common, and many people find they are carrying the virus only after births or going to hospitals for other reasons

“I don’t know what’s going to happen now,” the woman said through tears and an oxygen mask to aid her breathing after she was admitted with a possible tuberculosis infection, which her weakened immune system may have permitted.

Mexico provides antiretroviral drugs even to poor migrants without health insurance, but the challenge for them is reaching the cities where the drugs are dispensed. The transportation costs strain their budgets. Taking time off from work for doctor’s visits is another challenge.

The government has also slowly begun to acknowledge the problem, sending health workers into the countryside to visit returning migrants and teach them about the risks they face on the road. One program is called “Go Healthy, Return Healthy.”

Government health workers are focusing their prevention efforts not just on returning migrants but on those who intend to go. A variety of approaches have been used, from comic books to soap operas. The messages focus on the causes of AIDS, the benefits of condoms and the dangers of sharing needles

But the stigma surrounding AIDS in Mexico is such that even migrants who have contracted the virus dismiss the notion that extramarital affairs were a factor.

Another H.I.V.-positive migrant, a mother of three named Ana María who is now taking government-dispensed antiretroviral drugs, had gone to the United States with her husband and worked long hours in a fast-food restaurant and hotel in Chicago. She, too, found out she carried the virus after giving birth at a Chicago hospital

“Many people get infected there and then bring it back here,” said Ana María, who is in her early 40s. “I don’t know how we get infected but it could have been in the hospital there.” Her husband, who migrated with her and is also infected, nodded in agreement.

“I thought the crossing was the worst thing in my life,” she said, sitting on the front step of her home in a village outside of Puebla. “We saw human bones and clothes in the desert. There were robbers there, who would rape the girls and take all the money they could. I thought that had to be worst. Now, I have this.”