An emerging infectious disease (EID) is an infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human pathogens. EIDs are caused by newly identified species or strains (e.g. Severe acute respiratory syndrome, HIV/AIDS) that may have evolved from a known infection (e.g. influenza) or spread to a new population (e.g. West Nile fever) or to an area undergoing ecologic transformation (e.g. Lyme disease), or be "reemerging" infections, like drug resistant tuberculosis. Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and non-infectious conditions leading to the development of novel syndemics. Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.

The symptoms of an infection depend on the type of disease. Some signs of infection affect the whole body generally, such as fatigue, loss of appetite, weight loss, fevers, night sweats, chills, aches and pains. Others are specific to individual body parts, such as skin rashes, coughing, or a runny nose.

In certain cases, infectious diseases may be asymptomatic for much or even all of their course in a given host. In the latter case, the disease may only be defined as a "disease" (which by definition means an illness) in hosts who secondarily become ill after contact with an asymptomatic carrier. An infection is not synonymous with an infectious disease, as some infections do not cause illness in a host.

The mode of transmission of BoDV-1/2 is unclear but probably occurs through intranasal exposure to contaminated saliva or nasal secretions. Following infection, individuals may develop Borna disease, or may remain subclinical, possibly acting as a carrier of the virus.

Borna disease is an infectious neurological syndrome of warm-blooded animals, caused by Borna disease viruses 1 and 2 (BoDV-1/2), both of which are members of the species "Mammalian 1 bornavirus". BoDV-1 an 2 cause abnormal behaviour and fatality. Borna disease viruses 1 and 2 are neurotropic viruses and members of the "Bornaviridae" family within the "Mononegavirales" order.

Although Borna disease viruses 1 and 2 are mainly seen as the causative agent of Borna disease in horses and other animals, they are also controversially discussed as human infectious agents and therefore as potential zoonotic agents. The role of BoDV-1 and -2 in human illness is controversial and it is yet to be established whether BoDV-1 or -2 cause any overt disease in humans. However, correlative evidence exists linking BoDV-1/2 infection with neuropsychiatric disorders such as bipolar disorder.

Symptomatic infections are "apparent" and "clinical", whereas an infection that is active but does not produce noticeable symptoms may be called "inapparent," "silent," "subclinical", or . An infection that is inactive or dormant is called a "latent infection". An example of a latent bacterial infection is latent tuberculosis. Some viral infections can also be latent, examples of latent viral infections are any of those from the "Herpesviridae" family.

The word "infection" can denote any presence of a particular pathogen at all (no matter how little) but also is often used in a sense implying a "clinically apparent" infection (in other words, a case of infectious disease). This fact occasionally creates some ambiguity or prompts some usage discussion. To get around the usage annoyance, it is common for health professionals to speak of "colonization" (rather than "infection") when they mean that some of the pathogens are present but that no clinically apparent infection (no disease) is present.

A short-term infection is an "acute" infection. A long-term infection is a chronic infection. Infections can be further classified by causative agent (bacterial, viral, fungal, parasitic), and by the presence or absence of systemic symptoms (sepsis).

Porcine circoviral disease (PCVD) and Porcine circovirus associated disease (PCVAD), is a disease seen in domestic pigs. This disease causes illness in piglets, with clinical signs including progressive loss of body condition, visibly enlarged lymph nodes, difficulty in breathing, and sometimes diarrhea, pale skin, and jaundice. PCVD is very damaging to the pig-producing industry and has been reported worldwide. PCVD is caused by porcine circovirus type 2 (PCV-2).

The North American industry endorses "PCVAD" and European use "PCVD" to describe this disease.

Zoonoses are infectious diseases of animals (usually vertebrates) that can naturally be transmitted to humans.

Major modern diseases such as Ebola virus disease and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved to a separate human-only disease. Most strains of influenza that infect humans are human diseases, although many strains of swine and bird flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause pandemics such as the 1918 Spanish flu or the 2009 swine flu. "Taenia solium" infection is one of the neglected tropical diseases with public health and veterinary concern in endemic regions. Zoonoses can be caused by a range of disease pathogens such as viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in animals; however, only diseases that routinely involve animal to human transmission, like rabies, are considered direct zoonosis.

Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting infected. When humans infect animals, it is called reverse zoonosis or anthroponosis. The term is from Greek: ζῷον "zoon" "animal" and νόσος "nosos" "sickness".

A list of the more common and well-known diseases associated with infectious pathogens is provided and is not intended to be a complete listing.

Infections associated with diseases are those that are associated with possible infectious etiologies, that meet the requirements of Koch's postulates. Other methods of causation are described by the Bradford Hill criteria and Evidence-based medicine. Koch's postulates have been altered by some epidemiologists based upon sequence-based detection of distinctive pathogenic nucleic acid sequences in tissue samples. Using this method, absolute statements are not always possible regarding causation. Since this is true, higher amounts of distinctive pathogenic nucleic acid sequences would be in those exhibiting disease compared to controls since inoculating those without the pathogen is unethical. In addition, the DNA load should drop or become lower with the resolution of the disease. The distinctive pathogenic nucleic acid sequences load should also increase upon recurrence.

Other conditions are met to establish cause or association including studies in disease transmission. This means that there should be a high disease occurrence in those carrying an pathogen, evidence of a serologicalresponse to the pathogen, and the success of vaccination prevention. Direct visualization of the pathogen, the identification of different strains, immunological responses in the host, how the infection is spread and, the combination of these should all be taken into account to determine the probability that an infectious agent is the cause of the disease. A conclusive determination of a causal role of an infectious agent for in a particular disease using Koch's postulates is desired yet this might not be possible.

The leading cause of death worldwide is cardiovascular disease, but infectious diseases are the second leading cause of death worldwide and the leading cause of death in infants and children.

Zoonotic transmission can occur in any context in which there is companionistic (pets), economic (farming, etc.), predatory (hunting, butchering or consuming wild game) or research contact with or consumption of animals, animal products, or animal derivatives (vaccines, etc.).

An airborne disease is any disease that is caused by pathogens that can be transmitted through the air. Such diseases include many of considerable importance both in human and veterinary medicine. The relevant pathogens may be viruses, bacteria, or fungi, and they may be spread through breathing, talking, coughing, sneezing, raising of dust, spraying of liquids, toilet flushing or any activities which generates aerosol particles or droplets. Human airborne diseases do not include conditions caused by air pollution such as volatile organic compounds (VOCs), gasses and any airborne particles, though their study and prevention may help inform the science of airborne disease transmission.

Both PMWS and porcine dermatitis and nephropathy syndrome (PDNS) are associated to PCV-2. Many pigs affected by the circovirus also seem to develop secondary bacterial infections, like Glässer disease ("Haemophilus parasuis"), pulmonary pasteurellosis, colibacilosis, salmonellosis and others. Postmortem lesions occur in multiple organs, especially in lymphoid tissues and lung, giving rise to the term "multisystemic". Lesions may also affect the skin, kidney, reproductive tissue, brain, or blood vessels.

Wasting pigs is the most common sign of PMWS infection, increasing the mortality rate significantly.

Living fish afflicted with VHS may appear listless or limp, hang just

beneath the surface, or swim very abnormally, such as constant flashing

circling due to the tropism of the virus for the brain.

External signs may include darker coloration, exophthalmia ("pop eye"),

pale or red-dotted gills, sunken eyes, and bleeding around orbits (eye sockets) and

at base of fins.

Genetics researchers at the Lake Erie Research Center at the University of Toledo are developing a test that will speed diagnosis from a month to a matter of hours.

Acute: The acute form is a sudden onset of the disease at full-force. Symptoms include high fever, anemia (due to the breakdown of red blood cells), weakness, swelling of the lower abdomen and legs, weak pulse, and irregular heartbeat. The horse may die suddenly.

Subacute: A slower, less severe progression of the disease. Symptoms include recurrent fever, weight loss, an enlarged spleen (felt during a rectal examination), anemia, and swelling of the lower chest, abdominal wall, penile sheath, scrotum, and legs.

Chronic: The horse tires easily and is unsuitable for work. The horse may have a recurrent fever and anemia, and may relapse to the subacute or acute form even several years after the original attack.

A horse may also not appear to have any symptoms, yet still tests positive for EIA antibodies. Such a horse can still pass on the disease. According to most veterinarians, horses diagnosed EIA positive usually do not show any sign of sickness or disease.

EIA may cause abortion in pregnant mares. This may occur at any time during the pregnancy if there is a relapse when the virus enters the blood. Most infected mares will abort, however some give birth to healthy foals. Foals are not necessarily infected.

Studies indicate that there are breeds with a tolerance to EIA.

Recent studies in Brazil on living wild horses have shown that in the Pantanal, about 30% of domesticated and about 5.5% of the wild horses are chronically infected with EIA.

Coughing and rattling are common, most severe in young, such as broilers, and rapidly spreading in chickens confined or at proximity. Morbidity is 100% in non-vaccinated flocks. Mortality varies according to the virus strain (up to 60% in non-vaccinated flocks). Respiratory signs will subdue within two weeks. However, for some strains, a kidney infection may follow, causing mortality by toxemia. Younger chickens may die of tracheal occlusion by mucus (lower end) or by kidney failure. The infection may prolong in the cecal tonsils.

In laying hens, there can be transient respiratory signs, but mortality may be negligible. However, egg production drops sharply. A great percentage of produced eggs are misshapen and discolored. Many laid eggs have a thin or soft shell and poor albumen (watery), and are not marketable or proper for incubation. Normally-colored eggs, indicative of normal shells for instance in brown chickens, have a normal hatchability.

Egg yield curve may never return to normal. Milder strains may allow normal production after around eight weeks.

Equine infectious anemia or equine infectious anaemia (EIA), also known by horsemen as swamp fever, is a horse disease caused by a retrovirus and transmitted by bloodsucking insects. The virus ("EIAV") is endemic in the Americas, parts of Europe, the Middle and Far East, Russia, and South Africa. The virus is a lentivirus, like human immunodeficiency virus (HIV). Like HIV, EIA can be transmitted through blood, milk, and body secretions.

Transmission is primarily through biting flies, such as the horse-fly and deer-fly. The virus survives up to 4 hours in the vector (epidemiology). Contaminated surgical equipment and recycled needles and syringes, and bits can transmit the disease. Mares can transmit the disease to their foals via the placenta.

The risk of transmitting the disease is greatest when an infected horse is ill, as the blood levels of the virus are then highest.

Infectious pancreatic necrosis (IPN) is a severe viral disease of salmonid fish. It is caused by infectious pancreatic necrosis virus, which is a member of the Birnaviridae family. This disease mainly affects young salmonids, such as trout or salmon, of less than six months, although adult fish may carry the virus without showing symptoms. Resistance to infection develops more rapidly in warmer water. It is highly contagious and found worldwide, but some regions have managed to eradicate or greatly reduce the incidence of disease. The disease is normally spread horizontally via infected water, but spread also occurs vertically. It is not a zoonosis.

Marburg virus is a hemorrhagic fever virus of the "Filoviridae" family of viruses and a member of the species "Marburg marburgvirus", genus "Marburgvirus". Marburg virus (MARV) causes Marburg virus disease in humans and nonhuman primates, a form of viral hemorrhagic fever. Considered to be extremely dangerous, the WHO rates it as a Risk Group 4 Pathogen (requiring biosafety level 4-equivalent containment). In the United States, the NIH/National Institute of Allergy and Infectious Diseases ranks it as a Category A Priority Pathogen and the Centers for Disease Control and Prevention lists it as a Category A Bioterrorism Agent. It is also listed as a biological agent for export control by the Australia Group.

The virus can be transmitted by exposure to one species of fruit bats or it can be transmitted between people via body fluids through unprotected copulation and broken skin. The disease can cause bleeding (haemorrhage), fever and other symptoms much like Ebola. Funeral rituals are a particular risk. Actual treatment of the virus after infection is not possible but early, professional treatment of symptoms like dehydration considerably increase survival chances.

In 2009, expanded clinical trials of an Ebola and Marburg vaccine began in Kampala, Uganda.

Before puberty, the disease typically only produces flu-like symptoms, if any at all. When found, symptoms tend to be similar to those of common throat infections (mild pharyngitis, with or without tonsillitis).

VHSV is a hemorrhagic disease, meaning it causes bleeding. Internally,

the virus can cause petechial hemorrhaging (tiny spots of blood) in internal

muscle tissue, and petechial or severe hemorrhaging in internal organs and

other tissues. Internal hemorrhaging can be observed as red spots inside

a dead fish, particularly around the kidney, spleen, and intestines, as

well as the swim bladder, which would normally have a clear membrane.

The liver may be pale, mottled with red hyperemic areas, the kidney

may be swollen and unusually red, the spleen may be swollen, and the digestive

tract may be empty.

External signs are not always present, but if they are, hemorrhaging on the

skin's surface can appear as anywhere from tiny red dots (petechiae) to

large red patches.

The signs and symptoms of infectious mononucleosis vary with age.

The strongest evidence linking EBV and cancer formation is found in Burkitt's lymphoma

and nasopharyngeal carcinoma. Additionally, it has been postulated to be a trigger for a subset of chronic fatigue syndrome patients as well as multiple sclerosis and other autoimmune diseases.

Burkitt's lymphoma is a type of Non-Hodgkin's lymphoma and is most common in equatorial Africa and is co-existent with the presence of malaria. Malaria infection causes reduced immune surveillance of B cells immortalized by EBV, resulting in an excessive number of B cells and an increased likelihood of an unchecked mutation. Repeated mutations can lead to loss of cell-cycle control, causing excessive proliferation observed as Burkitt's lymphoma. Burkitt's lymphoma commonly affects the jaw bone, forming a huge tumor mass. It responds quickly to chemotherapy treatment, namely cyclophosphamide, but recurrence is common.

Other B cell lymphomas arise in immunocompromised patients such as those with AIDS or who have undergone organ transplantation with associated immunosuppression (Post-Transplant Lymphoproliferative Disorder (PTLPD)). Smooth muscle tumors are also associated with the virus in malignant patients.

Nasopharyngeal carcinoma is a cancer found in the upper respiratory tract, most commonly in the nasopharynx, and is linked to the EBV virus. It is found predominantly in Southern China and Africa, due to both genetic and environmental factors. It is much more common in people of Chinese ancestry (genetic), but is also linked to the Chinese diet of a high amount of smoked fish, which contain nitrosamines, well known carcinogens (environmental).

Feline infectious anemia (FIA) is an infectious disease found in felines, causing anemia and other symptoms. The disease is caused by a variety of infectious agents, most commonly "Mycoplasma haemofelis" (which used to be called "Haemobartonella"). "Haemobartonella" and "Eperythrozoon" species were reclassified as mycoplasmas. Coinfection often occurs with other infectious agents, including: feline leukemia virus (FeLV), feline immunodeficiency virus (FIV), "Ehrlichia" species, "Anaplasma phagocytophilum", and Candidatus "Mycoplasma haemominutum".

The period between infection and the first symptoms (incubation period) is typically 1–3 months in humans. Incubation periods as short as four days and longer than six years have been documented, depending on the location and severity of the contaminated wound and the amount of virus introduced. Initial signs and symptoms of rabies are often nonspecific such as fever and headache. As rabies progresses and causes inflammation of the brain and/or meninges, signs and symptoms can include slight or partial paralysis, anxiety, insomnia, confusion, agitation, abnormal behavior, paranoia, terror, and hallucinations, progressing to delirium, and coma. The person may also have hydrophobia.

Death usually occurs 2 to 10 days after first symptoms. Survival is rare once symptoms have presented, even with the administration of proper and intensive care. Jeanna Giese, who in 2004 was the first patient treated with the Milwaukee protocol, became the first person ever recorded to have survived rabies without receiving successful post-exposure prophylaxis. An intention-to-treat analysis has since found this protocol has a survival rate of about 8%.

The clinical presentation of prion diseases will vary from patient to patient. However, some general characteristics of prion diseases are listed below.