AIDS; 25 million from 1981 to present

Acquired immune deficiency syndrome or acquired immunodeficiency syndrome (AIDS) is a disease of the human immune system caused by the human immunodeficiency virus (HIV).
This condition progressively reduces the effectiveness of the immune system and leaves individuals susceptible to opportunistic infections and tumors. HIV is transmitted through direct contact of a mucous membrane or the bloodstream with a bodily fluid containing HIV, such as blood, semen, vaginal fluid, preseminal fluid, and breast milk.
This transmission can involve anal, vaginal or oral sex, blood transfusion, contaminated hypodermic needles, exchange between mother and baby during pregnancy, childbirth, breastfeeding or other exposure to one of the above bodily fluids.
AIDS is now a pandemic. In 2007, it was estimated that 33.2 million people lived with the disease worldwide, and that AIDS killed an estimated 2.1 million people, including 330,000 children. Over three-quarters of these deaths occurred in sub-Saharan Africa, retarding economic growth and destroying human capital.
Genetic research indicates that HIV originated in west-central Africa during the late nineteenth or early twentieth century. AIDS was first recognized by the U.S. Centers for Disease Control and Prevention in 1981 and its cause, HIV, identified in the early 1980s.
Although treatments for AIDS and HIV can slow the course of the disease, there is currently no vaccine or cure. Antiretroviral treatment reduces both the mortality and the morbidity of HIV infection, but these drugs are expensive and routine access to antiretroviral medication is not available in all countries. Due to the difficulty in treating HIV infection, preventing infection is a key aim in controlling the AIDS pandemic, with health organizations promoting safe sex and needle-exchange programmes in attempts to slow the spread of the virus.

AIDS is a mix of infections and complications as a result of progressive damage to the body’s immune system caused by HIV. AIDS is now considered a pandemic.

Influenza; 36,000 deaths annually

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae (the influenza viruses), that affects birds and mammals. The name influenza is Italian and means "influence" (Latin: influentia). The most common symptoms of the disease are chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort. Sore throat, fever and coughs are the most frequent symptoms. In more serious cases, influenza causes pneumonia, which can be fatal, particularly for the young and the elderly. Although it is often confused with other influenza-like illnesses, especially the common cold, influenza is a much more severe disease than the common cold and is caused by a different type of virus. Influenza may produce nausea and vomiting, particularly in children, but these symptoms are more common in the unrelated gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu".
Typically, influenza is transmitted through the air by coughs or sneezes, creating aerosols containing the virus. Influenza can also be transmitted by direct contact with bird droppings or nasal secretions, or through contact with contaminated surfaces. Airborne aerosols have been thought to cause most infections, although which means of transmission is most important is not absolutely clear. Influenza viruses can be inactivated by sunlight, disinfectants and detergents. As the virus can be inactivated by soap, frequent hand washing reduces the risk of infection.
Influenza spreads around the world in seasonal epidemics, resulting in the deaths of hundreds of thousands worldwide annually, and millions in pandemic years. On average 41,400 people died each year in the United States between 1979 and 2001 from influenza. Three influenza pandemics occurred in the 20th century and killed tens of millions of people, with each of these pandemics being caused by the appearance of a new strain of the virus in humans. Often, these new strains appear when an existing flu virus spreads to humans from other animal species, or when an existing human strain picks up new genes from a virus that usually infects birds or pigs. An avian strain named H5N1 raised the concern of a new influenza pandemic, after it emerged in Asia in the 1990s, but it has not evolved to a form that spreads easily between people. In April 2009 a novel flu strain evolved that combined genes from human, pig, and bird flu, initially dubbed "swine flu" and also known as influenza A/H1N1, emerged in Mexico, the United States, and several other nations. The World Health Organization officially declared the outbreak to be a "pandemic" on June 11, 2009 (see 2009 flu pandemic). The WHO's declaration of a pandemic level 6 was an indication of spread, not severity.

Influenza, which is more commonly known as flu, is a highly infectious disease that is caused by influenza virus. Transmission of the disease is by airborne and through physical contact.

Spanish Flu, 1918-19; 100 million deaths

The 1918 flu pandemic (commonly referred to as the Spanish Flu) was an influenza pandemic that spread to nearly every part of the world. It was caused by an unusually virulent and deadly influenza A virus strain of subtype H1N1. Historical and epidemiological data are inadequate to identify the geographic origin of the virus. Most of its victims were healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients. The flu pandemic has also been implicated in the sudden outbreak of encephalitis lethargica in the 1920s.
The pandemic lasted from March 1918 to June 1920, spreading even to the Arctic and remote Pacific islands. It is estimated that anywhere from 50 to 100 million people were killed worldwide which is from three to seven times the casualties of the First World War (15 million).An estimated 50 million people, about 3% of the world's population (approximately 1.6 billion at the time), died of the disease. An estimated 500 million, or 1/3 were infected.
Scientists have used tissue samples from frozen victims to reproduce the virus for study. Given the strain's extreme virulence there has been controversy regarding the wisdom of such research. Among the conclusions of this research is that the virus kills via a cytokine storm (overreaction of the body's immune system) which explains its unusually severe nature and the concentrated age profile of its victims. The strong immune systems of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults caused fewer deaths.

The flu pandemic that happened in 1918 is termed as a category 5 flu pandemic which was caused by the flu virus strain A with subtype H1N1.

Bubonic Plague; 250 million deaths

Bubonic plague is the best known manifestation of the bacterial disease plague, caused by the Gram-negative bacterium Yersinia pestis (formerly known as Pasteurella pestis). It belongs to the family Enterobacteriaceae. The term "bubonic plague" was often used synonymously for plague, but it does in fact refer specifically to an infection that enters through the skin and travels through the lymphatics, as is often seen in flea-borne infections. Bubonic plague kills about half of infected patients in 3–7 days without treatment, and may be the Black Death that swept through Europe in the 1340s, killing tens of millions.

This disease outbreak was mainly caused by fleas and rodents infected with Xenopsylla cheopsis. Humans were infected after being bitten by an affected rodent.

Malaria; 2.7 million deaths annually

Malaria is a vector-borne infectious disease caused by a eukaryotic protist of the genus Plasmodium. It is widespread in tropical and subtropical regions, including parts of the Americas, Asia, and Africa. Each year, there are approximately 350–500 million cases of malaria, killing between one and three million people, the majority of whom are young children in Sub-Saharan Africa. Ninety percent of malaria-related deaths occur in Sub-Saharan Africa. Malaria is commonly associated with poverty, but is also a cause of poverty and a major hindrance to economic development.
Malaria is one of the most common infectious diseases and an enormous public health problem. Five species of the plasmodium parasite can infect humans; the most serious forms of the disease are caused by Plasmodium falciparum. Malaria caused by Plasmodium vivax, Plasmodium ovale and Plasmodium malariae causes milder disease in humans that is not generally fatal. A fifth species, Plasmodium knowlesi, causes malaria in macaques but can also infect humans. This group of human-pathogenic Plasmodium species is usually referred to as malaria parasites.
Usually, people get malaria by being bitten by an infective female Anopheles mosquito. Only Anopheles mosquitoes can transmit malaria, and they must have been infected through a previous blood meal taken on an infected person. When a mosquito bites an infected person, a small amount of blood is taken, which contains microscopic malaria parasites. About one week later, when the mosquito takes its next blood meal, these parasites mix with the mosquito's saliva and are injected into the person being bitten. The parasites multiply within red blood cells, causing symptoms that include symptoms of anemia (light-headedness, shortness of breath, tachycardia, etc.), as well as other general symptoms such as fever, chills, nausea, flu-like illness, and, in severe cases, coma, and death. Malaria transmission can be reduced by preventing mosquito bites with mosquito nets and insect repellents, or by mosquito control measures such as spraying insecticides inside houses and draining standing water where mosquitoes lay their eggs. Work has been done on malaria vaccines with limited success and more exotic controls, such as genetic manipulation of mosquitoes to make them resistant to the parasite have also been considered.
Although some are under development, no vaccine is currently available for malaria that provides a high level of protection; preventive drugs must be taken continuously to reduce the risk of infection. These prophylactic drug treatments are often too expensive for most people living in endemic areas. Most adults from endemic areas have a degree of long-term infection, which tends to recur, and also possess partial immunity (resistance); the resistance reduces with time, and such adults may become susceptible to severe malaria if they have spent a significant amount of time in non-endemic areas. They are strongly recommended to take full precautions if they return to an endemic area. Malaria infections are treated through the use of antimalarial drugs, such as quinine or artemisinin derivatives. However, parasites have evolved to be resistant to many of these drugs. Therefore, in some areas of the world, only a few drugs remain as effective treatments for malaria.

Malaria is an infectious disease which is vector-borne. The causative agent is the protozoan parasites. It is a common disease in the sub-tropics and tropical regions.

Ebola; 160,000 deaths from 2000 to present

Ebola virus: A notoriously deadly virus that causes fearsome symptoms, the most prominent being high fever and massive internal bleeding. Ebola virus kills as many as 90% of the people it infects. It is one of the viruses that is capable of causing hemorrhagic (bloody) fever.

Epidemics of Ebola virus have occurred mainly in African countries including Zaire (now the Democratic Republic of Congo), Gabon, Uganda, the Ivory Coast, and Sudan. Ebola virus is a hazard to laboratory workers and, for that matter, anyone who is exposed to it.

Infection with Ebola virus in humans is incidental -- humans do not "carry" the virus. The way in which the virus first appears in a human at the start of an outbreak has not been determined. However, it has been hypothesized that the first patient (the index case) becomes infected through contact with an infected animal.

Ebola virus is transmitted by contact with blood, feces or body fluids from an infected person or by direct contact with the virus, as in a laboratory. People can be exposed to Ebola virus from direct contact with the blood or secretions of an infected person. This is why the virus has often been spread through the families and friends of infected persons: in the course of feeding, holding, or otherwise caring for them, family members and friends would come into close contact with such secretions. People can also be exposed to Ebola virus through contact with objects, such as needles, that have been contaminated with infected secretions.

The Ebola virus was first isolated in1976 from the dual outbreaks that occurred in Zaire and Sudan. It is a zoonotic disease as it affects lowland apes as well as humans.

Cholera; 12,000 deaths from 1991 to present

Cholera is a bacterial disease usually spread through contaminated water. Cholera causes severe diarrhea and dehydration. Left untreated, cholera can be fatal in a matter of hours.

Modern sewage and water treatment have virtually eliminated cholera in industrialized countries. The last major outbreak in the United States occurred in 1911. But cholera is still present in Asia, the Middle East, Latin America, India and sub-Saharan Africa. The risk of cholera epidemic is highest when poverty, war or natural disasters force people to live in crowded conditions without adequate sanitation.

Cholera is easily treated. Death results from severe dehydration that can be prevented with a simple and inexpensive rehydration solution.

The epidemic or Asiatic cholera is a very serious type of diarrheal ailment caused by Vibrio cholera. The mode of transmission is by ingesting contaminated food and water.

Smallpox; Population drop from 12 million to 235,000

Smallpox is a contagious, disfiguring and often deadly disease caused by the variola virus. Smallpox is believed to have been around for thousands of years. Few other illnesses have had such a devastating effect on human health and history.

There's no treatment or cure for smallpox. The only prevention is vaccination.

Naturally occurring smallpox was eradicated worldwide by 1980 — the result of an unprecedented immunization campaign. However, interest in the disease remains high because stocks of smallpox virus, kept for research purposes, are stored in two high-security labs — one in the United States and one in Siberia. In addition, it's suspected that other governments or entities may hold stockpiles. This has lead to concerns that smallpox may be used as a biological warfare agent.

Smallpox is a highly contagious viral disease that has two variants. The V major has a 35% mortality rate while the less severe V minor has a 1% mortality rate.

Polio, 10,000 deaths from 1916 to present

Polio is a contagious viral illness that in its most severe form causes paralysis, difficulty breathing and sometimes death.

In the U.S., the last case of wild polio — polio caused naturally, not by a vaccine containing live virus — occurred in 1979. Today, despite a concerted global eradication campaign, wild poliovirus continues to affect children and adults in Afghanistan, India, Nigeria and Pakistan.

The Centers for Disease Control and Prevention (CDC) advises taking precautions to protect against polio if you're traveling anywhere there's a risk of polio. If you're a previously vaccinated adult who plans to travel to an area where polio is occurring, you should receive a booster dose of inactivated poliovirus. Immunity following a booster dose lasts a lifetime.

Polio or infantile paralysis is a viral disease that is transmitted through the fecal-oral course.

Black Death; 75 million deaths

The Black Death
1347 - 1350

Culprit: Oriental Rat Flea

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Family: Pulicidae
Order: Siphonaptera
Genus: Xenopsylla
Species: cheopis

Dead littered the streets everywhere. Cattle and livestock roamed the country unattended. Brother deserted brother.

The Black Death was one of the worst natural disasters in history. In 1347 A.D., a great plague swept over Europe, ravaged cities causing widespread hysteria and death. One third of the population of Europe died. "The impact upon the future of England was greater than upon any other European country." (Cartwright, 1991) The primary culprits in transmitting this disease were oriental rat fleas carried on the back of black rats.

The Black Plague is the one of the most serious pandemic outbreak in modern history. It was believed to have started in Central Asia and affected Europe during the 13th century.