Ciudad Hidalgo and Tecun Uman are two sides of the same city--the former in Mexico, the latter in Guatemala. All that separates them are the muddy waters of the River Suchiate, and the International Bridge that spans it. A few cars, some European missionaries and a handful of pedestrians slowly cross the concrete span. A Mexican immigration officer, his shirt unbuttoned to expose a large golden cross, diligently scrutinizes passports. But the Army soldiers stationed at this crossing are idle. One of them, sitting on the edge of the bridge with a cigarette, his rifle slung over his shoulder, peers down at the river. There, in the shadows of the International Bridge and for miles in either direction, an unending stream of people--some in makeshift rafts, others wading on foot--move between the countries with the nonchalance of jaywalkers.

Mexico's 2,000-mile-long border with the United States is a heavily guarded focal point of the global argument about immigration. But Mexico's southern boundary gets little attention, and has now become a major problem. Hundreds of thousands of poor Central Americans cross into Mexico each year on a desperate quest for a more prosperous life. Some are lucky and wend their way, illegally, to the United States. But many of the migrants get waylaid long before, often in Mexico's Chiapas state just inside the border. They are robbed or attacked by Central American gangs, known as maras, or by Mexican police whose job is to protect them. Many of the Central Americans have unprotected sex in border towns. Some are rape victims; others sleep with truckers in exchange for a ride north; still others are sold into sex slavery. Rampant crime and corruption, combined with drug use and the thriving sex trade, have turned the area into a lawless hinterland where the threat of HIV looms large.

The crisis has sparked criticism of Mexico's lax health, human-rights and immigration policies. "[Mexico] is in many cases much nastier to the people from Central America than the U.S. is to people from Mexico," says former foreign minister Jorge Castaneda. The situation is so dire that Chiapas Public Security Secretary Horacio Schroeder last week called the rise of gang activity and the attacks on migrants on the southern border an issue of "national security," and announced a stepped-up military presence.

The biggest problem, say Mexican health experts, is that heavy migration across the country's southern and northern borders is rapidly spreading the HIV virus. President Vicente Fox has called for more AIDS awareness in border towns, and tried to tackle police corruption. The more unsafe migratory conditions become, say health experts, the faster the virus is believed to spread. But a severe lack of funds, and the lingering stigma of AIDS, have crippled prevention programs. About 30 percent of new HIV cases in Mexico originate in the high-migration states of Jalisco, Michoacan and Zacatecas. Mexican officials maintain that migrants returning from the United States pose the greatest HIV risk to the country's rural population. The AIDS rate in America is twice that of Mexico. But HIV infection rates in Central America can be up to six times higher than in Mexico--a worrisome statistic given the large numbers of migrants who move back and forth every day. Experts call that pattern "circular migration," and note that it has contributed to some of the world's worst outbreaks of AIDS. "All the conditions are in place for an explosion of AIDS in Mexico," says Mario Bronfman, former director of Mexico's National Institute of Public Health. "The scenario is the same as in Africa."

With little money, many migrants trade sex for passage. "The highest rates of HIV we've seen are along the migrant [truck] routes," says Jorge Saavedra, head of CenSida, Mexico's official AIDS office. Bronfman estimates that some 80 percent of women--and an unknown but growing number of men--who migrate through Chiapas on their way to the United States have engaged in some sort of sex work along the way. The expectation that sex will be part of the migration process is so commonplace now among Central American women that many of them take an anti-pregnancy injection called Berlotal before entering Mexico. Few bring condoms with them on their journey. "These people have absolutely nothing left to lose," says the Rev. Flor Marillo, an Italian priest who runs the House of Migrants in the town of Tapachula, about half an hour from the Guatemalan border. "They are the kamikazes of poverty and they're willing to die to get north."

Maria-Ines Aguilar Osuna, 28, an HIV-positive patient living in Tijuana's only free AIDS clinic, is one such person. Osuna got to Tijuana only after selling sex for rides with, by her own count, 28 truck drivers along a well-traveled route that originates along the Suchiate. When she got to Tijuana, she tested positive for HIV. A young gay man living in Ciudad Hidalgo, referring to himself as a "client" of the migrants, says he has repeatedly offered food, shelter and money in exchange for sex. "I have told them that if they let me penetrate them, without a condom, I'll give them more money," he says. "They do whatever you want. The last thing they're thinking about is protection."

The situation is hardly better in any one of the many "tolerance zones" that dot the ghettos of Mexico's border towns--so called because the bars and brothels are loosely regulated at best. Doctors occasionally visit the 15 or so brothels in Ciudad Hidalgo, testing the sex workers for STDs. But when NEWSWEEK visited a bar called El Dragon Rojo (The Red Dragon), with several government health workers, none of the three Central American prostitutes working there had any condoms. The owner of the bar sells the women condoms for 10 pesos (about $1) each, an exorbitantly high price. To make matters worse, men regularly offer as much as five times the price of a standard trick if the women agree not to use condoms. Of the three women, two had plans to go north, once they had earned enough money. "Sex is the only passport some of these people have," says Marillo.

Migrants make easy targets for gangs. In Tecun Uman, on the Guatemalan side of the Suchiate, men walk down the street in broad daylight with pistols tucked under their shirts. Tattooed youth from the maras of El Salvador and Honduras make their home on the banks of the river, terrorizing migrants. Last March Wilmer Dubon, 25, a Honduran immigrant, was on his way to Veracruz, clinging to the back of a freight train. A band of maras boarded the train and threw him off. He fell under one of the wheels of the train and his left leg was cut off above the knee. He lay bleeding in a field for eight hours before he was rescued. Now he spends his days with some two dozen other amputees at the Albergue de Jesus del Buen Pastor, a refuge in Tapachula for migrants with injuries. Last December, Mexico unofficially joined Honduras, El Salvador and Nicaragua in declaring war against the maras. In the past two months security officials in Chiapas have arrested about 90 suspected gang members.

Border residents say the authorities themselves are involved in extortion, sex trafficking and violence against migrants. Chiapas Bishop Felipe Arizmendi Esquivel recently warned that the municipal and state police of Chiapas were the "principal" figures in a spate of crimes. And though several policemen have been jailed in Ciudad Hidalgo after migrants brought complaints against them, the abuses continue. "A majority of the attacks we've seen down here are from the police," says Gabriela Coutino of Guatemala's National Institute for Migration, in Tapachula. Her institute provides the only migrant-protection service in the country, a small group known as "Betas" who patrol the countryside in bright orange trucks providing advice, and sometimes protection, to migrants.

Experts say there are no easy solutions to Mexico's border crisis. Central American migration is driven by abject poverty that governments in the region have been unable to resolve. Migration to the United States from Mexico and Central America actually increased after the 9/11 attacks, despite tighter border-security measures. That fact has led analysts to conclude that any attempt by Mexico to clamp down on migrants from the south might actually increase the risks to which they are exposed. The more desperate people are, the more vulnerable. That is a scary thought given the burgeoning HIV problem. "I would not hesitate to call it a ticking time bomb," says Silvio Martinelli, the Central American coordinator for UNAIDS in Guatemala.

Last week in Ciudad Hidalgo, a cargo train heading north into Mexico slowly pulled away from the town's dilapidated station. Suddenly hundreds of migrants--young men, women with babies, even an elderly couple--emerged from the shadows and began to run alongside the train. As it gathered speed, most jumped on board. But according to one person who stayed behind, danger awaited the group. Gangs were waiting at the next bridge. Had they been tipped off about the threat, most of the migrants would surely have hopped on the train anyway. Until a desperately poor region can create more economic opportunities for its people, they will continue to risk their lives.