A handful of deadly infectious diseases claim millions
of lives worldwide each year: lower respiratory tract
infections, diarrheal diseases
, and malaria
. Together, they account for nearly one-fifth of deaths globally. Several of these diseases have plagued humankind throughout history, often decimating populations with greater efficiency than wars. In an age of vaccines
, and dramatic scientific progress, these diseases should have been brought under control. Yet they continue to kill at an alarming rate, particularly in the developing world. In low-income countries the dominant causes of death are infectious and parasitic diseases (including malaria) and poor medical care surrounding childbirth. By contrast, in high-income countries the leading causes of death are noncommunicable diseases, such as heart disease and cancer. Infectious and parasitic causes of mortality are farther down the list.
Several of these diseases have plagued humankind throughout history, often decimating populations with greater efficiency than wars.
Lower respiratory tract infections (including pneumonia) account for more than 4 million deaths worldwide each year—the greatest global killer among infectious diseases. Pneumonia is also the leading cause of death of the very young, often striking children with low birth weight or those whose immune systems are weakened by malnutrition or other diseases. Most of these deaths occur in developing countries.
Diarrheal diseases are the second-leading cause of infectious disease deaths worldwide, accounting for more than 2 million deaths annually, and nearly one-fifth of all deaths of children under the age of five. These infections are so widespread in developing countries that parents often fail to recognize when symptoms become critical. Children die simply because their bodies are weakened—often through rapid loss of fluids and undernourishment. The burden of diarrheal diseases is highest in deprived areas where there is poor sanitation, inadequate hygiene, and unsafe drinking water.
HIV stands for human immunodeficiency virus, the virus that causes acquired immunodeficiency syndrome (AIDS), the final stage of HIV infection. HIV appears to have jumped to humans early in the 20th century from a type of chimpanzee in West Africa—most likely when humans hunted these animals for meat and came into contact with their infected blood. The virus slowly spread across Africa and later to other parts of the world.
Unlike most other viruses, HIV attacks the immune system, destroying a type of white blood cell (T cells or CD4 cells) that the immune system needs to fight disease.
In 2008 more than 33 million people worldwide were infected with HIV and an estimated 2 million died. Unlike most other viruses, HIV attacks the immune system, destroying a type of white blood cell
(T cells or CD4 cells) that the immune system needs to fight disease. HIV is transmitted by having sex with someone infected with HIV, by sharing needles and syringes with an infected person, through blood or blood product transfusions, or by being exposed as a fetus or an infant to the virus before or during birth or through breastfeeding. HIV is not transmitted through casual contact, such as shaking hands, hugging, modest kissing, or drinking from the same glass.
Someone in the world is newly infected with the tuberculosis (TB) bacterium every second. In 2008 there were an estimated 9.4 million new cases of tuberculosis and 1.8 million deaths. The vast majority of TB deaths are in the developing world, and more than half of all deaths occur in Asia. In the United States, 12,904 TB cases were reported in 2008, a nearly 3 percent decline from the number reported in 2007.
TB, which is caused by the bacterium Mycobacterium tuberculosis,
is a contagious disease that spreads through the air when an infected person coughs or sneezes and people nearby breathe in the bacteria. TB bacteria can live in the body without making an individual sick, a condition known as latent TB infection. More than 2 billion people—about one- third of the world’s population—are infected, many asymptomatically, with TB bacilli
. A person with active TB can be treated by taking several drugs for 6 to 12 months.
Multidrug-resistant TB (MDR-TB) is a new threat. It is difficult and expensive to treat and fails to respond to the standard first-line drugs that are most easily tolerated and have historically worked best against the disease. Extensively drug-resistant TB (XDR-TB) occurs when resistance to second-line drugs, which are typically more toxic and less effective, develops on top of MDR-TB. Such infections are highly lethal.
Each year as many as 300 million people are infected with malaria worldwide, and up to 1 million die, most of them young children in sub-Saharan Africa.
On the global stage, HIV/AIDS and TB are tragically interconnected. Among HIV-infected people whose immune systems are weakened by the effects of the virus, the risk of developing TB is much higher than for persons with normal immune systems. Indeed, TB is the leading cause of death worldwide among people infected with HIV. Likewise, among people with latent (inactive) TB infection, HIV infection is the strongest known risk factor for progressing to active TB disease.
Like the other global killers, malaria
occurs mostly in poor, tropical, and subtropical areas of the world. Each year as many as 300 million people are infected with malaria worldwide, and up to 1 million die, most of them young children in sub-Saharan Africa. Malaria is a mosquito-borne disease caused by several different protozoan
parasites. Humans infected with malaria parasites, depending on the type, can develop a wide range of illnesses, from mild infection that does not produce symptoms to the classic symptoms of malaria (fever, chills, sweating, headaches, muscle pains) to severe complications (anemia
, kidney failure, coma) that can lead to death.
Malaria serves as a particularly dramatic example of infectious disease re-emergence and illustrates the challenges of controlling human vector
-borne diseases. Following the drastic depopulation of Anopheles
mosquitoes in the first half of the 20th century due to liberal use of the pesticide DDT, malaria began its resurgence in Asia in the late 1960s. In Sri Lanka, where only 17 cases of malaria were reported in 1963, a major epidemic of more than 440,000 cases erupted 5 years later, after preventive vector control was replaced with a strategy of identifying and treating new cases of the illness. Similarly, by the mid-1970s millions of new cases had occurred in India after mosquito eradication efforts ceased. In Africa, where vector control programs were never initiated, a more recent upsurge in infections, including several explosive epidemics, has erupted in endemic
areas. A number of factors appear to be driving this global resurgence, including the rapid spread of drug resistance among malarial parasites, changing rainfall patterns, and water development projects, such as dams, which create new mosquito breeding sites.
Yet despite today’s dire headlines, progress is being made against the world’s modern infectious scourges. The scale-up of antiretroviral therapy has reduced the number of AIDS deaths and mother-to-child transmissions, and has improved survival and productivity. Wider access to antiretroviral treatments has also been accompanied by a dramatic reduction in prices. The prevalence of TB has declined since 2000, partly because the WHO’s Directly Observed Therapy Short-Course strategy brought treatment and a cure to tens of millions of patients. And malaria deaths have fallen with the development of artemisinin-based drugs, distribution of insecticide-treated bed nets, and indoor residual spraying of insecticides. Alongside these efforts there have been major investments in health care systems—bolstering infrastructure, laboratories, and human resources.