Many think of bubonic plague, leprosy and polio as diseases of the past -- things that might have had a part in history, but aren't around any longer. But these diseases are still very much with us.
The pneumonic and bubonic plagues are caused by the bacterium Yersinia pestis, the difference being that pneumonic plague can spread from person to person -- without infected fleas.
The pneumonic form struck China earlier this year, and bubonic plague still persists in the United States, in the Southwest. Wild rats and fleas carry the disease, and when in proximity to humans, fleas will spread it to them rather than simply among rats.
Polio is still a problem in many countries. It can cause paralysis, in the legs and even the lungs.
But more often it has only subtle signs, so it is difficult to determine who has the virus, making it tricky to eradicate. Complicating matters is that symptoms of polio can often mimic those of other viruses.
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.
About 8 to 9 million people in Latin America are infected, and 400,000 people in the United States have the disease. 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.
Leprosy is among the oldest of human diseases. The disease attacks skin and ultimately nerve cells. It is caused by a bacterium similar to the one responsible for tuberculosis.
The most common cause within the United States is exposure to armadillos, whether through eating them, having a farm of them or hunting them.
Hookworm still plagues many living in rural poverty throughout the world. The hookworm parasite, which lives in soil, causes severe anemia in its victim and the infection can prove difficult to get rid of.
Tuberculosis is still around, even if it isn't as deadly as it once was. But TB still poses a major problem for doctors. It's an insidious, slow-onset disease. Ultimately, patients can develop a persistent fever and lose energy and weight. Patients may not know they have the disease, and they can spread it to other people during that time.
Tuberculosis has become more of a threat than it once was with the development of forms that are resistant to the drugs typically used to treat the disease. Because of this, tuberculosis is still fatal to 40 to 50 percent of people who catch it.