It has crossed oceans and continents; it stalks the world's most marginal people as they struggle to survive.
K. Sangeetha's husband brought HIV/AIDS home to their village of Gangaikondacholapuram on a rickety bus from Chennai, the coastal city better known to many as Madras.
Each day, poor men like him from villages throughout the world's second most populous country head to the booming cities to find work. The road from Sangeetha's village to Chennai to the north is about 175 miles; along the way laborers find work in quarries, brick plants or sugar cane fields.
When they go, the men stay for months at a time, leaving their wives and children as they try to earn enough money to pull their families out of the poverty of subsistence farming. Many of them also encounter prostitutes while they are away from home and contract HIV. Many, like Sangeetha's husband, die.
India has surpassed South Africa as the country with the largest number of people living with HIV/AIDS. As of last year, there were an estimated 5.7 million of them.
India's government is fighting back. It is offering free antiretroviral drugs in the larger cities. A local organization has enlisted barbers in the fight: They hand out free condoms and comic books to educate men about the disease.
Rural people have heard that there is hope if they can get to the cities. But for many of them, it still is out of reach.
For some who have been stricken, the seven-hour bus ride to Chennai and hours of standing in line for a month's supply of drugs are too difficult. For others, the $6 cost of the bus ticket is too much.
So they stay home, often stigmatized by their neighbors, left to confront the certainty of decline and death in the same isolation in which they lived.
Sangeetha, a 35-year-old widow with a 15-year-old daughter, was one such woman.
She died this fall.