"You could stop breathing," Hotez said. "That's why during polio epidemics in the United States, they used to put people in iron lungs."
But, he noted, "It's only one out of 100 that are infected with polio that actually develop paralysis. Many people would have no symptoms at all."
To eradicate smallpox, doctors used what is known as "ring vaccination," giving the vaccine to anyone who lived around a victim of the disease.
But because polio has only subtle signs, it is much more difficult to determine who has the virus. Complicating matters, Hotez said, is that symptoms of polio can often mimic those of other viruses.
"You have to be much more aggressive in how you do surveillance for the virus," he said.
So the ambiguity and the subtlety of polio make it a trickier target than smallpox.
A similar effect can take place with pertussis, or whooping cough, simply because the immunity from the vaccine given can wear off over time and the disease does not affect adults as harshly as it does children.
"Many people who were immunized as children may no longer be immune as adults," Markel said.
If they become infected, they can pass that infection to children, who tend to have more serious cases of whooping cough than adults.
Until a few years ago, Chagas disease was not really found in the United States. But conditions in post-Katrina Louisiana and immigration from areas where the disease is endemic have brought more cases to the United States.
The disease is spread by the bug known as the kissing bug and the assassin bug. It bites its human victim, defecating and causing an itch, which becomes Chagas disease when the victim scratches the area, allowing the infection to enter the body.
"Chagas disease is a parasitic infection. It's a disease of poverty," said Dr. Carlos Franco-Paredes, who treats infectious diseases at Atlanta clinics through Emory University. The bug typically lives in poor quality housing, feasting on host victims at night.
Hotez notes that 8 million to 9 million people in Latin America are infected, and he estimates that 400,000 people in the United States have the disease. Patients who get a blood transfusion may be infected, although a test was created in 2006, and Franco-Paredes noted that it is possible to get the infection by drinking some juices in South America, since the sugar cane used is the nesting place of the kissing bug.
The infection attacks the heart, but over the course of many years. Five percent of people will develop symptoms early on, such as liver or spleen enlargement.
But the other 95 percent will show no signs for 20 or 30 years.
"You have heart failure, or you have dilation of the esophagus or the colon," Franco-Paredes said.
People will lose the ability to eat.
"There's really no treatment of Chagas after you have developed all those complications," he said.
Hotez notes one treatment is possible. "Once it gets beyond a certain point, the only treatment is heart transplantation," he said, but once the disease reaches that stage it becomes chronic and incurable.
"You have to catch it early," Hotez said.
Franco-Paredes notes that a patient needs heart treatment or a pacemaker, but patients who would get Chagas disease, for the most part, could never afford the treatments.
Calling the disease "a biological expression for social inequality," he said, "If there's not redistribution of wealth in Latin America, we won't be able to eliminate Chagas disease."