A plague that wipes out 25 million people in three years. A disease that kills its victims within hours of the symptoms appearing. A flu that makes its sufferers turn blue before drowning in their own lungs.

It sounds like the stuff of science-fiction movies, but for centuries, these horrid diseases were very real (in fact, some of them could still post a threat today). What are they? Read on as we uncover the seven deadliest diseases in history.

1. Smallpox

By some accounts, smallpox is considered to have killed more people than any other infectious disease. However, thanks to the discovery of the smallpox vaccine, the last known naturally contracted case of the disease was in 1977, according to the World Health Organization (WHO). During its deadliest years, this highly contagious disease killed as many of 30 percent of people who contracted it. Those who survived were often blinded or marked with deep scars. Despite its eradication, smallpox may still pose a threat today, if released as a biological weapon.

2. Spanish Flu

In 1918, the so-called Spanish flu struck quickly and catastrophically. The first reports of the illness came from Spain, but it affected countries around the globe. Some experts put the death toll for that single year as high as 50 million people worldwide. Patients usually took on a bluish hue hours before dying, most likely due to insufficient oxygen. Autopsies revealed that their lungs filled with fluid, causing a drowning-like death. Unlike most types of influenza, which hit children and the elderly hardest, this strain proved deadly even for young adults. Scientists continue to study the Spanish flu to this day, trying to determine exactly what made it so deadly and if it could happen again.

3. The Black Plague

Also known as the black death, this is the plague that kept coming back. At its most deadly, the black plague is thought to have killed 25 million people in Europe—about a third of the population—from 1347 to 1350. The high death toll from the black plague, so named for the black boils it left on the body, is believed to have actually been a result of three similar illnesses: bubonic, septicaemic, and pneumonic plagues. The scourge swept through Europe, killing millions more, on other occasions throughout the next several centuries. However, in no instance did its severity match that of the black death of the mid-14th century.

4. Tuberculosis

The 1905 Nobel Prize in Medicine went to Robert Koch for "his investigations and discoveries" relating to tuberculosis (TB), according to the Nobel Foundation. However, more than 100 years later, TB still kills nearly 2 million people a year and is ranked as the eighth leading cause of death worldwide by the WHO. Symptoms including severe coughing, fever, chills, and fatigue. People with weakened immune systems are especially susceptible to TB; in fact, it's the number-one killer of AIDS patients. In 2007, Andrew Speaker, an Atlanta lawyer and TB patient, made headlines for flying from Europe to Canada, despite being instructed not to do so for fear of infecting fellow passengers.

5. Malaria

Caused by a parasite and transmitted when a person is bitten by an infected mosquito, Malaria remains a serious problem in parts of Africa, although it has been nearly eradicated in the United States. According to the Centers for Disease Control and Prevention (CDC), more than 1 million people die from it annually, and as many as 500 million people are infected each year. The flu-like symptoms appear after 10 to 15 days after being bitten. Malaria can be treated with medication, if caught soon enough.

6. Ebola

This virus is a relatively new deadly disease that has been known to kill up to 90 percent of its victims. It first appeared in 1976, in Sudan and Zaire. It can be transmitted from person to person or by handling materials from an infected animal. Its early symptoms include fever, headache, backache, vomiting, and diarrhea; eventually it can cause inflammation and swelling of nearly all major organs. Most people die from shock, when their bodies stop getting enough blood flow.

7. Cholera

Cases of cholera are rare today, due to advancements in water treatment and sewage systems. However, in the 19th and even early 20th centuries, cholera epidemics struck several times. The disease, which is characterized by watery diarrhea, can kill a healthy person as soon as two to three hours after the onset of symptoms, though it usually takes several days. Although cholera is still present today in parts of the world that have been ravaged by war or famine, it generally doesn't pose a problem when clean water and proper sanitation is available. A vaccine, which lasts for up to six months, is available for people traveling to areas where cholera may be a concern, though some experts question its necessity and effectiveness.