INTRODUCTION
An infectious disease is a disease that is caused by the invasion of a host by agents whose activities harm the host's tissues and can be transmitted to other individuals.

Infectious disease have for centuries ranked with wars and famine as major challenges to human progress and survival. They remain among the leading cause of death and disability worldwide. Against a constant background of established infections, epidemics of new and old infectious diseases periodically emerge, greatly magnifying the global burden of infections. Study of these infections reveal the evolutionary properties of pathogenic microorganisms and the dynamic relationships between microorganisms, their host and the environment.

Emerging and Re-emerging Infectious Diseases
At least a dozen "new" diseases have been identified (such as AIDS, Legionnaire disease, Zika virus and hantavirus pulmonary syndrome), and traditional diseases that appeared to be "on their way out" (such as malaria and tuberculosis) are resurging. Globally, infectious diseases remain the leading cause of death, and they are the third leading cause of death in the United States. Clearly, the battle has not been won.

Emerging infectious diseases are diseases that (1) have not occurred in humans before (this type of emergence is difficult to establish and is probably rare); (2) have occurred previously but affected only small numbers of people in isolated places (AIDS and Ebola hemorrhagic fever are examples); or (3) have occurred throughout human history but have only recently been recognized as distinct diseases due to an infectious agent.

Re-emerging infectious diseases are diseases that once were major health problems globally or in a particular country, and then declined dramatically, but are again becoming health problems for a significant proportion of the population (malaria and tuberculosis are examples). Many specialists in infectious diseases include re-emerging diseases as a subcategory of emerging diseases. (1)

HISTORY
Emerging infections have been familiar threats since ancient times. They were once called lionos and later as pestilences, pests and plagues.  Some  examples can be sited like black death  and 1918 influenza pandemic among many.  The age of discovery, starting in fifteenth century was a particularly a disastrous period with regard to spread of infectious diseases. Importation of small pox in Mexico caused 10-25 million deaths in 1520-1521. Other American Indian and Pacific civilizations were destroyed by imported small pox and measles.

For centuries, mankind seemed helpless against these sudden epidemics. With the establishment of germ theory and the identification of specific microbes as the causative agent of wide variety if infectious diseases led to enormous progress, notably the development of vaccination and subsequently antimicrobials.  By 1950s, which witnessed the wide spread use of  penicillin, the development of polio vaccines and the discovery of drugs for tuberculosis, complacency had set in and the optimists at the end of 19th century predicted  end of infectious disease era.

Some experts remained skeptical, aware of recurrent lessons from history. They were less persuaded by successes than alarmed by failures such as the lack of progress against infections in the developing world and the global spread of antimicrobial resistance. Emergence of AIDS- one of history’s most devastating pandemics, led to renewed appreciation of the inevitability and consequences of emerging infectious diseases.  In past 25 years, some of the factors that resulted in AIDS have also led to re-emergence of historically important diseases such as cholera, diphtheria, trench fever, plague, influenza and most recently Zika virus. (2)

PUBLIC HEALTH CHALLENGES

Global distribution and prevalence
Emerging and re-emerging infectious diseases are more often found in developing and under developed countries. Their impact on morbidity and mortality is more severe in these populations and particularly on infants and children.  About 15 million of 57 million annual deaths worldwide are estimated to be related directly to infectious diseases.

Challenges
Infectious diseases will continue to emerge and re-emerge leading to unpredictable epidemics and difficult challenges to public health. Surveillance and response, the key element in controlling emerging infectious diseases, depend on rapid clinical diagnosis and detection and contamination in populations and the environment. Globally, such efforts are coordinated by the world health organization. However, for developing countries economic constrain poses a great challenge for prevention and control of infectious diseases.

CAUSING AGENTS
There are five major types of infectious agents: bacteria, viruses, fungi, protozoa, helminthes and newly recognized prions. A brief review of the general characteristics of viral infections and examples of some diseases they cause follows.

Viruses
Microbiologists have found viruses that infect all organisms, from plants and animals to fungi and bacteria. Viruses, however, are not organisms themselves because, apart from a host cell, they have no metabolism and cannot reproduce.

Just like other infectious agents, viruses cause disease by disrupting normal cell function. They do this in a variety of ways. Some viruses make repressor proteins that stop the synthesis of the host cell's proteins, RNA, and DNA. Viral activity may weaken cell membranes and lysosomal membranes, leading to cell autolysis. Some viral proteins are toxic to cells, and the body's immune defenses also may kill virus-infected cells.

Viruses are classified using a variety of criteria, including shape, size, and type of genome. Among the DNA viruses are the herpes viruses that cause chicken pox, cold sores, and painful genital lesions, and the poxvirus that causes smallpox. Significant RNA viruses that cause human disease include rhinoviruses that cause most common colds; myxoviruses and paramyxoviruses that cause influenza, measles, and mumps; rotaviruses that cause gastroenteritis; and the retroviruses that cause AIDS and several types of cancer. (3)

FACTORS INFLUENCING THEIR FREQUENT REOCCURANCE

A number of factors contribute towards emergence and re-emergence of infectious diseased these include but are not limited to the following:

1. Disease reservoir. The reservoir for a disease is the site where the infectious agent survives. Animals often serve as reservoirs for these diseases that infect humans. The major reservoir for Yersinia pestis, the bacteria that causes plague, is wild rodents. There are also nonliving reservoirs. Since the reservoir animal can be completely eliminated in the environment, the disease is able to survive and re-emerge for example in case of Ebola.

2. Easy mode of transmission. Infectious agents may be transmitted through either direct or indirect contact. Direct contact occurs when an individual is infected by contact with the reservoir, for example, by touching an infected person, ingesting infected meat, or being bitten by an infected animal or insect. Transmission by direct contact also includes inhaling the infectious agent in droplets emitted by sneezing or coughing and contracting the infectious agent through intimate sexual contact. This makes the infection to easily transmitted between individuals.

3. Mutation and antimicrobial resistance. With times most of these infectious agents are able to mutate and acquire antimicrobial resistant genes. This has posed a great challenge to development of newer and stronger antimicrobial agents. Antimicrobial resistance contributes to great extend towards survival of emerging and re-emerging infections.

CONTROL METHODS

Developed countries have regulations that help protect the general public from infectious diseases. Public health measures typically involve eliminating the pathogen from its reservoir or from its route of transmission. Those measures include ensuring a safe water supply, effectively managing sewage treatment and disposal, and initiating food safety, animal control, and vaccination programs.

Safe water
Many pathogens that cause gastrointestinal diseases (for example, those that cause cholera and typhoid fever) are transmitted via water. Travelers to developing countries are frequently advised to be immunized against these diseases. There fore, water purification methods should be applied.

Because gastrointestinal pathogens typically leave the body in the feces, public water must be guarded against contamination from sewage. Municipal water is usually tested for the presence of coliform organisms (nonpathogenic microorganisms that are part of the normal flora of the gastrointestinal tract) as indicators of sewage contamination.

Sewage treatment and disposal
Sewage includes wash water, water from toilets, and storm run-off. These fluids may carry the pathogens for many waterborne diseases, including giardiasis and hepatitis A; therefore, to ensure public safety the U.S. government (and the governments of other developed countries) requires that sewage be treated to eliminate pathogens.

Food safety programs
In many developing and the developed country like the United States has many standards, inspection plans, and regulations about food preparation, handling, and distribution. Meat-packing facilities are inspected regularly to detect and eliminate diseased animals, ensure that standards for processes such as meat cutting and refrigeration are observed, and detect residues from pesticides and antibiotics as well as contamination by bacteria and other parasites. Restaurants and supermarkets are similarly inspected.

Animal control programs
Animals are carriers of many diseases that also affect humans. Inspecting domestic herd animals for tuberculosis (due to the bacterium Mycobacterium bovis) and brucellosis (a disease that causes spontaneous abortion in domestic herd animals and abscesses of the liver, spleen, bone marrow, and lymph nodes in humans) has helped eliminate the threat of passing the pathogens for those diseases to humans in contaminated milk and meat. Many diseases, including bubonic plague, are spread by rodents, and rat control, especially in urban areas, is a major component of public health efforts. Insects also transmit many diseases (a notable example is malaria). The spread of insect-borne diseases can be controlled by eliminating breeding areas for insects (for example, draining areas where stagnant water collects) and using pesticides. Many imported animals must be tested for specific diseases to prevent the introduction of those diseases into the country. (4)

Vaccination programs
The value of immunization for an individual's health is obvious; however, it is also important for public health. If a certain proportion of a population (called the threshold proportion) is immune to a disease, the pathogen that causes that disease will be unable to reproduce itself at a high enough level to maintain itself in the population. This is because once the infected host recovers or dies, there will not be enough new, susceptible hosts for the pathogen to infect. Eventually, the pathogen cannot spread any further and could be eliminated from the population. Even if elimination of the pathogen does not occur, there will be relatively few cases of the related disease and epidemics of the disease in the population will be avoided. This phenomenon is called herd immunity.

The threshold proportion varies depending on the disease and other conditions in the relevant population. Vaccination programs led by public health officials aim to achieve the immunization of at least the threshold number of individuals for the population. (5)

CONCLUSION

Emerging and re-emerging infectious diseases remain among the leading cause of death and disability worldwide. Against a constant background of established infections, epidemics of new and old infectious diseases periodically emerge, greatly magnifying the global burden of infections. Study of these infections reveal the evolutionary properties of pathogenic microorganisms and the dynamic relationships between microorganisms, their host and the environment.

They are the cause of serious global health problems.

This paper examined the trends of emerging and re-emerging infectious disease. and is examines (i) history of emerging and re-emerging infectious diseases, (ii) public health challenges, (iii) causing agents (iv)factors and control methods.

The reasons for the development of a control strategy based on population intervention rather than on individual treatment are discussed. An efficient vaccination program in most affected populations can reduce the morbidity and mortality caused by these emerging and re-emerging infectious diseases.

                                                                                          
REFERENCE:

1. David M. Morens, Gregory K. Folkers and Anthony S. Fauci, National institute of Allergy and infectious diseases, 2004 Nature Publishing Group.

2. Anderson RM, May RM. Infectious diseases of humans: Dynamics and control. New York: Oxford University Press; 1992.

3. Biological Sciences Curriculum Study. Teaching tools. Dubuque, IA: Kendall/Hunt Publishing Company; 1999.

4. Bonwell CC, Eison JA. Active learning: Creating excitement in the classroom. Washington, DC: The George Washington University: School of Education and Human Development; 1991. (ASHE-ERIC Higher Education Report No. 1)

5. Brody CM. Collaborative or cooperative learning? Complementary practices for instructional reform. The Journal of Staff, Program, & Organizational Development. 1995;12(3):134–143.