LA GLORIA, Mexico (CNN) -- Tucked away in this small mountain village in Mexico, off a dusty road flanked by pig farms, is where the earliest case of swine flu -- a virus spreading globally -- was confirmed.
Five-year-old Edgar Hernandez, known as "patient zero," survived the earliest documented case of swine flu.
Meet the child known as "patient zero" by his doctors -- 5-year-old Edgar Hernandez, who survived the earliest documented case of swine flu in an outbreak that, officials say, has now spread across four continents.
His family lives in the 3,000-population village of La Gloria in the state of Veracruz, where a flu outbreak was reported on April 2. State officials arrived and tested dozens of people.
Lab tests confirmed that Edgar was the only patient in Veracruz to test positive for the swine flu virus; the others had contracted a common flu. Health officials had returned to Edgar's sample only after cases of the new flu strain were spotted around the country. Watch Dr. Gupta meet little Edgar »
"In this case, there's a patient who turned out to be positive for the swine-flu virus, with the exception that at that time in no region of the world it had been established as an etiological, epidemic cause," said Mexico Health Secretary Jose Angel Cordova.
Edgar has managed to bounce back from his symptoms and playfully credits ice cream for helping him feel better.
His mother blamed a huge pig farm in the neighborhood for the virus.
Officials have conducted tests at the farm owned by U.S. company Smithfield Foods, and those tests came back negative.
Swine influenza, or flu, is a contagious respiratory disease that affects pigs. When the flu spreads person to person, instead of from animals to humans, it can continue to mutate, making it harder to treat or fight, because people have no natural immunity. Symptoms include fever, runny nose, sore throat, nausea, vomiting and diarrhea. Learn more about swine flu and how to treat it »
Common seasonal flu kills 250,000 to 500,000 people every year worldwide, far more than the current outbreak of swine flu. But there is no vaccine for this new disease, and little natural immunity, an expert said.
"I think the reason to be concerned is ... we had a vaccine for regular flu," said Dr. Carlos del Rio of the Emory University School of Medicine in Atlanta, Georgia. "This is a totally new virus. ... You have a virus to which there's no pre-vaccination, there's no prior immunity. And, therefore, the mortality rate may be higher than other influenza viruses." Watch why swine flu is a "sloppy virus" »
Researchers do not know how the virus is jumping relatively easily from person to person, or why it's affecting what should be society's healthiest demographic.
Meanwhile, in hard-hit Mexico City, the government closed universities, postponed sporting events, asked restaurants to serve only take-out food, and canceled church services in an effort to try to stem the spread of the virus.
Worried residents continue to flood in night and day at hospitals, only to be turned away by armed guards. Watch Dr. Sanjay Gupta inside a Mexico City hospital »
Two of the most common antiviral drugs, Tamiflu and Relenza -- which are effective against swine flu -- are in short supply in Mexico's capital. It also has become impossible to find protective surgical masks, which the government had handed out to one out of every five residents.
"I was looking for a mask at my local pharmacy, but they sold out," supermarket worker Rafael Martinez said as he rode the subway. "I know it's a risk, but I can't find one."
CNN Chief Medical Correspondent Dr. Sanjay Gupta in La Gloria, Mexico, contributed to this report.