Epidemiology

In 2010, about 10 million people died of infectious diseases.

The World Health Organization collects information on global deaths by International Classification of Disease (ICD) code categories. The following table lists the top infectious disease by number of deaths in 2002. 1993 data is included for comparison.

Worldwide mortality due to infectious diseases

Rank	Cause of death	Deaths 2002 (in millions)	Percentage of all deaths	Deaths 1993 (in millions)	1993 Rank
N/A	All infectious diseases	14.7	25.9%	16.4	32.2%
1	Lower respiratory infections	3.9	6.9%	4.1	1
2	HIV/AIDS	2.8	4.9%	0.7	7
3	Diarrheal diseases	1.8	3.2%	3.0	2
4	Tuberculosis (TB)	1.6	2.7%	2.7	3
5	Malaria	1.3	2.2%	2.0	4
6	Measles	0.6	1.1%	1.1	5
7	Pertussis	0.29	0.5%	0.36	7
8	Tetanus	0.21	0.4%	0.15	12
9	Meningitis	0.17	0.3%	0.25	8
10	Syphilis	0.16	0.3%	0.19	11
11	Hepatitis B	0.10	0.2%	0.93	6
12–17	Tropical diseases (6)	0.13	0.2%	0.53	9, 10, 16–18
Note: Other causes of death include maternal and perinatal conditions (5.2%), nutritional deficiencies (0.9%), noncommunicable conditions (58.8%), and injuries (9.1%).

The top three single agent/disease killers are HIV/AIDS, TB and malaria. While the number of deaths due to nearly every disease have decreased, deaths due to HIV/AIDS have increased fourfold. Childhood diseases include pertussis, poliomyelitis, diphtheria, measles and tetanus. Children also make up a large percentage of lower respiratory and diarrheal deaths. In 2012, approximately 3.1 million people have died due to lower respiratory infections, making it the number 4 leading cause of death in the world.

Historic pandemicsedit

With their potential for unpredictable and explosive impacts, infectious diseases have been major actors in human history. A pandemic (or global epidemic) is a disease that affects people over an extensive geographical area. For example:

• Plague of Justinian, from 541 to 542, killed between 50% and 60% of Europe's population.
• The Black Death of 1347 to 1352 killed 25 million in Europe over 5 years. The plague reduced the old world population from an estimated 450 million to between 350 and 375 million in the 14th century.
• The introduction of smallpox, measles, and typhus to the areas of Central and South America by European explorers during the 15th and 16th centuries caused pandemics among the native inhabitants. Between 1518 and 1568 disease pandemics are said to have caused the population of Mexico to fall from 20 million to 3 million.
• The first European influenza epidemic occurred between 1556 and 1560, with an estimated mortality rate of 20%.
• Smallpox killed an estimated 60 million Europeans during the 18th century (approximately 400,000 per year). Up to 30% of those infected, including 80% of the children under 5 years of age, died from the disease, and one-third of the survivors went blind.
• In the 19th century, tuberculosis killed an estimated one-quarter of the adult population of Europe; by 1918 one in six deaths in France were still caused by TB.
• The Influenza Pandemic of 1918 (or the Spanish flu) killed 25–50 million people (about 2% of world population of 1.7 billion). Today Influenza kills about 250,000 to 500,000 worldwide each year.

Emerging diseasesedit

In most cases, microorganisms live in harmony with their hosts via mutual or commensal interactions. Diseases can emerge when existing parasites become pathogenic or when new pathogenic parasites enter a new host.

1. Coevolution between parasite and host can lead to hosts becoming resistant to the parasites or the parasites may evolve greater virulence, leading to immunopathological disease.
2. Human activity is involved with many emerging infectious diseases, such as environmental change enabling a parasite to occupy new niches. When that happens, a pathogen that had been confined to a remote habitat has a wider distribution and possibly a new host organism. Parasites jumping from nonhuman to human hosts are known as zoonoses. Under disease invasion, when a parasite invades a new host species, it may become pathogenic in the new host.

Several human activities have led to the emergence of zoonotic human pathogens, including viruses, bacteria, protozoa, and rickettsia, and spread of vector-borne diseases, see also globalization and disease and wildlife disease:

• Encroachment on wildlife habitats. The construction of new villages and housing developments in rural areas force animals to live in dense populations, creating opportunities for microbes to mutate and emerge.
• Changes in agriculture. The introduction of new crops attracts new crop pests and the microbes they carry to farming communities, exposing people to unfamiliar diseases.
• The destruction of rain forests. As countries make use of their rain forests, by building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring previously unknown microorganisms.
• Uncontrolled urbanization. The rapid growth of cities in many developing countries tends to concentrate large numbers of people into crowded areas with poor sanitation. These conditions foster transmission of contagious diseases.
• Modern transport. Ships and other cargo carriers often harbor unintended "passengers", that can spread diseases to faraway destinations. While with international jet-airplane travel, people infected with a disease can carry it to distant lands, or home to their families, before their first symptoms appear.