It includes outbreaks of previously unknown diseases or known diseases whose incidence in humans has significantly increased in the past two decades.₃

These are the known diseases that have reappeared after a significant decline in incidence.₃

At least 30 previously unknown diseases have emerged since 1973.₅ Some important ones are:

Figure 4

Human demographic: Ever increasing world population and migration of masses in search of job to urban areas, leading to overcrowding, inadequate sanitation and hygiene, which provide an ideal breeding ground for infectious agents.

Human behavior: Changing family structure, risky sexual activity, drug abuse, outdoor recreation, changing eating patterns, have all increased the spread of infectious agents. Imprudent use of microbial drugs, decreased compliance with vaccination policy, use of deadly pathogens such as small pox and anthrax, as agents of bioterrorism, have all led to re-emergence of diseases which were thought to be controlled previously e.g. HIV/AIDS, tuberculosis, malaria.

International travel & commerce: Increased international travel, especially without taking appropriate vaccine and other protective measures, lead to increased infection in travelers, brining infection back home. Increased commerce through increased import of food materials from developing to developed countries lead to increase in food-borne diseases in developed countries. Moreover, increased trade in exotic animals for pets and food sources has contributed to rise in opportunity for pathogens to jump from animal reservoir to humans e.g. monkey pox.

Changes in genetic make-up: Natural genetic variations, re-combinations and adaptations allow ‘new' strains of known pathogens to appear. e.g. influenza.

Land use: Inappropriate land use and irrigation projects, upset local ecology and creates new habitats. Encroachment of human civilization on the environment disturbs delicate balance which humans share with the microbes. Diseases like lyme disease has emerged as a result of disturbance of this balance.

Health infrastructure: Inappropriate and inadequate health infrastructure, especially in developing countries has been unable to cope with increasing demands.

In addition to above networks, following networks are also there for surveillance of various emerging and re-emerging diseases:

Outbreak Verification: Outbreak verification system was established by WHO in 1997. After receiving report of the disease outbreak WHO investigates them to confirm its public health importance and exact nature. An outbreak verification team is formed for the purpose.

Research:₂,₆ Various organizations like CDC, National Institute of Allergy & Infectious Diseases (NIAID), National Institute of Diabetes (NIDDK), Digestive and Kidney diseases, has made research contributions in field of HIV/AIDS, Tuberculosis, Malaria, Hepatitis C etc. Advances in genomics, proteomics have helped in better understanding of pathogenesis, host community and drug resistance and are helping in identifying new drug targets and develop new vaccines and diagnostics.₂ Progress in synthetic chemistry, robotics, computer, modeling and developments in molecular and genetic epidemiology are all helping to understand the pathogens, host factors, transmission patterns.₇