Infectious diseases are illnesses that are caused by infection with a disease-causing organism, and can often be passed from one person to another. Infectious diseases vary greatly in severity, with symptoms that are mild and inconvenient, like the common cold, or those that are life-threatening, such as HIV/AIDS. Other familiar infectious diseases include chicken pox, meningitis, and streptococcal infection. Food poisoning, or foodborne illness, caused by salmonella, E. coli, and others, fall into this class of diseases. Symptoms of infectious diseases can be acute (short in duration) or chronic (long-lasting).
Persons with infectious diseases usually appear to be sick, although it is possible to harbor certain infectious diseases without having symptoms. The pathogens, or disease-causing agents, responsible for most infectious diseases include bacteria, viruses, parasites, protozoa, prions, or fungi. Infectious diseases are also known as communicable diseases or transmissible diseases.
Deaths from infectious diseases are decreasing worldwide due to vaccines, effective treatments, and prevention measures. However, infectious diseases still result in more than 14 million deaths per year worldwide, and account for about one-fourth of all human deaths. HIV/AIDS, tuberculosis, and malaria are the single diseases that claim the lives of the most people, although respiratory infections and diarrhea-type diseases also claim over 5 million lives per year. Although anyone can get an infectious disease, children are especially susceptible to many, and efforts to control outbreaks of infectious diseases often center on children. Many children in industrialized countries are inoculated against once-common childhood illnesses such as measles, mumps, chicken pox, rubella, and whooping cough, among others. As a result, the overall incidence of some of these diseases, including measles and polio, have dramatically decreased. Respiratory diseases, diarrhea, tuberculosis, malaria, and HIV/AIDS, however, continue to grip the world’s especially vulnerable population of children less than five years of age.
Most infectious diseases can be transmitted from one person to another by either direct or indirect means. The viruses that cause colds, influenza, and measles, for example, are usually transmitted directly by inhaling infectious droplets that are scattered into the air when an infected person coughs or sneezes. The HIV virus that causes AIDS is usually transmitted by direct contact with infected blood or body fluids, often during sex. Infection with methicillin-resistant Staphylococcus aureus (MRSa) bacteria is often transmitted in the community by indirect means, such as when bacteria from an infected person contaminates exercise equipment in a gym. The equipment is then used by another person who contracts the bacteria. In this case, the object that harbored the bacteria and served as a vehicle for transmission is known as a fomite.
The parasite that causes malaria is normally transmitted between humans indirectly by a mosquito, who first gains the parasite when it bites and obtains a blood meal from an infected human, then harbors the parasite, and finally transfers the parasite via its salivary glands when it bites another human. In this case, the mosquito that enables transmission of the disease is known as a vector. Hepatitis A, polio, and diarrheal diseases are most often transmitted by direct contact with the causative agent that is present in feces. Usually, the pathogen enters the mouth via contaminated hands or by eating contaminated food.
Handwashing is the single most effective measure for preventing the spread of infectious diseases. Access to clean water for both drinking and sanitation are also a top priority for preventing a wide range of infectious diseases. According to the United Nations (UN), almost one billion people worldwide do not have access to safe, clean drinking water, and over 2.5 billion people, or about 40 percent of the world’s population, do not have access to adequate sanitation. In 2013, the UN claimed that access to sanitation and clean drinking water could save the lives of over 1.6 million young children each year by reducing waterborne infectious diseases, and expanded its Universal Declaration of Human Rights to include access to clean water as a basic and essential human right worldwide.
On the personal level, maintaining good hygiene, receiving routine immunizations, avoiding crowds during flu season, maintaining a good exercise pattern and diet for overall health, practicing safe sex, covering the mouth when sneezing or coughing, seeking the advice of a health care provider when illness occurs, and completing all prescribed medications are all responsible measures that help limit both the exposure and transmission of infectious diseases. At the doctor’s office, physicians and other healthcare providers are urged to prescribe antibiotics only for appropriate bacterial infections, as overuse of antibiotics has been linked to the emergence of strains of bacteria that cause difficult-to-treat infections that show resistance to common antibiotics.
In developed countries, lack of participation in immunization programs can lead to infectious disease outbreaks. In 2015, Norway experienced a jump in reported cases of mumps ("kusma" in Norwegian) in college students. Public heath officials attributed the spike in cases to parental resistance to Norway's Childhood Immunization Program, and incomplete adherence to recommended vaccination schedules (two doses of MMR vaccine at 15 months and a booster at 11 years) that ultimately results in unvaccinated college age adults mixing with unvaccinated and infected foreign students. There is an option in Norway to not take vaccines, thus increasing the potential for outbreaks to occur.
Reporting and tracking disease outbreaks
Physicians in the United States report the occurrence of infectious diseases such as pertussis (whooping cough), tuberculosis, shigellosis, and hepatitis A, B, and C, along with other diseases that have been determined to have public health importance to state health officials. State officials investigate local occurrences of infectious diseases and in turn, notify federal public health officials working at the National Institutes of Health and the U.S. Centers for Disease Control and Prevention (CDC) of outbreaks. Globally, the World Health Organization (WHO) coordinates this intricate web of surveillance and reporting of infectious diseases. Prompt surveillance and reporting was credited with halting the epidemic of severe acute respiratory syndrome (SARS) in 2003, when it suddenly emerged in China and quickly spread from Asia to North America via infected air travelers. China was initially criticized for withholding vital information about the epidemic, but later shared information with the WHO, and has since pledged active cooperation in the international effort to identify and track emerging infectious diseases.
Health officials in London, United Kingdom, identified a new SARS-like coronavirus in a patient transferred from Qatar in September 2012. In March 2013, Chinese health authorities and the WHO identified a new type of avian influenza A subtype H7N9 that first occurred around Shanghai, China.
Because infectious diseases such as pandemic influenza, HIV/AIDS, and resistant strains of tuberculosis, malaria, gonorrhea, and E. coli, among others, have emerged or re-emerged, new and urgent challenges exist for scientists who investigate and fight infectious diseases. Infectious diseases currently incur such a burden that they threaten to destroy social order in some developing nations, and pose extremely difficult to public health problems even in the wealthiest societies. Globalization has brought about demand for legal and illegal trade in animals and animal products that present disease risks. Air travel has created the situation in which travelers can return home from areas where diseases are endemic (naturally occurring) within the incubation period of every infectious disease, which can potentially precipitate an epidemic. Increasing habitat destruction and human population growth have brought wild animals and humans in closer contact, facilitating the transmission of zoonotic diseases, those that are transmitted from animals. Climate change and warming trends are also encouraging the populations of disease vectors, including mosquitoes and ticks, to increase in areas where they were previously not viable.
Despite these challenges, some victories against infectious disease have been among the most important achievements in human public health. Smallpox is eradicated among humans, and the virus exists only in a few carefully controlled laboratory settings. Measles cases have been reduced by more than 70 percent worldwide in the last three decades. Polio, once on the verge of eradication, has maintained a stubborn hold in a few African and Asian countries, and renewed international efforts are again under way to eliminate the disease. New drugs to fight resistant tuberculosis and for pre-exposure prevention of HIV infection are under development, as are new vaccines for infectious diseases that are especially prevalent in young children. Significantly, a new era of alliance between governments, industry, private foundations, and non-governmental organizations is occurring, dedicated and coordinated in their efforts to make headway against the burden of infectious diseases in the world.
Special health challenges
In areas of civil unrest or war, sanitation-related disease and infectious disease outbreaks can become deadly. In the evolving revolution and civil war in Libya during 2011, for example, refugee encampments—especially near airports and border areas—filled with fleeing workers and their families. In the Libyan capital of Tripoli, for example, people seeking to leave Libya congregated near the airport, creating a large unorganized camp without sanitation. Unofficial reports from doctors operating in the area described open excrement running through areas used for cooking and washing. Physicians on the ground also report daily deaths from infectious disease that in some areas surpassed the number of causalities attributed to fighting.
In December 2015, the WHO identified the most dire epidemic disease threats, defined as those diseases requiring urgent prioritization in research and development, including Crimean Congo hemorrhagic fever (CCHF), Ebola virus disease (EVD), Lassa hemorrhagic virus disease (LVHD), Marburg virus disease (MVD), Middle East respiratory syndrome (MERS-CoV), severe acute respiratory syndrome (SARS), Nipah virus disease (NiVD), and Rift Valley fever (RVFD). Listed as serious, but less urgent, diseases requiring additional study were Chikungunya virus disease (CHIVD), severe fever with thrombocytopenia syndrome (SFTS), and Zika virus disease (ZIKVD). Diseases with epidemic potential but not listed as urgent because major research and clinical treatment networks already exist include acquired immune deficiency syndrome (AIDS) and human immunodeficiency virus (HIV) infection, avian influenza, dengue hemorrhagic fever, malaria, and tuberculosis.
As the Zika virus continued a rapid advance in the Americas, World Health Organization (WHO) officials declared the virus and its suspected link to birth defects an international public health emergency. By September 2016, more than one million cases had been reported in South America, Central America, the Caribbean, and in the Southern United States. As of May 2017, 59 countries and territories around the world have confirmed vector-borne transmission of Zika virus. An additional 19 countries are considered at lower risk, with no Zika transmission within the last three months, but with historical transmission since 2007. The WHO expects that Zika infections might eventually spread freely in all but two countries in the Americas, Chile, and Canada, where populations of the mosquito vector may be kept below levels necessary to cause a severe outbreak.
In May 2017, The World Health Organization declared a new Ebola outbreak in the Democratic Republic of Congo. Three people died following reports of a cluster of nine "undiagnosed illnesses" with symptoms close to those of Ebola-like hemorrhagic fever. Laboratory tests did not initially link the outbreak to 2014 epidemic in West Africa that killed more than 11,300 people in Guinea, Sierra Leone, Liberia, and neighboring countries.