The U.S. Centers for Disease Control and Prevention (CDC) publishes a journal "Emerging Infectious Diseases" that identifies the following factors contributing to disease emergence:

- Microbial adaption; e.g. genetic drift and genetic shift in Influenza A

- Changing human susceptibility; e.g. mass immunocompromisation with HIV/AIDS

- Climate and weather; e.g. diseases with zoonotic vectors such as West Nile Disease (transmitted by mosquitoes) are moving further from the tropics as the climate warms

- Change in human demographics and trade; e.g. rapid travel enabled SARS to rapidly propagate around the globe

- Economic development; e.g. use of antibiotics to increase meat yield of farmed cows leads to antibiotic resistance

- Breakdown of public health; e.g. the current situation in Zimbabwe

- Poverty and social inequality; e.g. tuberculosis is primarily a problem in low-income areas

- War and famine

- Bioterrorism; e.g. 2001 Anthrax attacks

- Dam and irrigation system construction; e.g. malaria and other mosquito borne diseases

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.

Other causes or associations of disease are: a compromised immune system, environmental toxins, radiation exposure, diet and lifestyle choices, stress, and genetics. Diseases may also be multifactorial, requiring multiple factors to induce disease. For example: in a murine model, Crohn's disease can be precipitated by a norovirus, but only when both a specific gene variant is present and a certain toxin has damaged the gut.

Infectious diseases are sometimes called contagious disease when they are easily transmitted by contact with an ill person or their secretions (e.g., influenza). Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted. Other types of infectious/transmissible/communicable diseases with more specialized routes of infection, such as vector transmission or sexual transmission, are usually not regarded as "contagious", and often do not require medical isolation (sometimes loosely called quarantine) of victims. However, this specialized connotation of the word "contagious" and "contagious disease" (easy transmissibility) is not always respected in popular use.

Infectious diseases are commonly transmitted from person to person through direct contact. The types of contact are through person to person and droplet spread. Indirect contact such as airborne transmission, contaminated objects, food and drinking water, animal person contact, animal reservoirs, insect bites, and environmental reservoirs are another way infectious diseases are transmitted,

Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microorganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani releases a toxin that paralyzes muscles, and staphylococcus releases toxins that produce shock and sepsis. Not all infectious agents cause disease in all hosts. For example, less than 5% of individuals infected with polio develop disease. On the other hand, some infectious agents are highly virulent. The prion causing mad cow disease and Creutzfeldt–Jakob disease invariably kills all animals and people that are infected.

Persistent infections occur because the body is unable to clear the organism after the initial infection. Persistent infections are characterized by the continual presence of the infectious organism, often as latent infection with occasional recurrent relapses of active infection. There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise.

Persistent infections cause millions of deaths globally each year. Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.

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.

Any age may be affected although it is most common in children aged five to fifteen years. By the time adulthood is reached about half the population will have become immune following infection at some time in their past. Outbreaks can arise especially in nursery schools, preschools, and elementary schools. Infection is an occupational risk for school and day-care personnel. There is no vaccine available for human parvovirus B19, though attempts have been made to develop one.

Postweaning multisystemic wasting syndrome ("PMWS") is the classic PCVD entity, caused by PCV-2. PCV-2 has a near universal distribution – present in most pig herds. In contrast, PMWS is more sporadic in its distribution. Experimental induction of PMWS has not been achieved by PCV-2 infection alone, using infectious DNA clones of the virus or a pure form of PCV-2 derived from infectious DNA clones. Therefore, it is assumed that PMWS is a multifactorial disease. PCV-2 is necessary but not sufficient for the development of PMWS. However, viral infection by itself tends to cause only mild disease, and co-factors such as other infections or immunostimulation seem necessary for development of severe disease.[1] For example, concurrent infection with porcine parvovirus or PRRS virus, or immunostimulation lead to increased replication of PCV-2 and more severe disease in PCV-2-infected pigs. There is no significant correlation of the disease with virus sequence variation with affected and control pigs.

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.

An airborne disease can be caused by exposure to a source: an infected patient or animal, by being transferred from the infected person or animal’s mouth, nose, cut, or needle puncture. People receive the disease through a portal of entry: mouth, nose, cut, or needle puncture.

Airborne transmission of disease depends on several physical variables endemic to the infectious particle. Environmental factors influence the efficacy of airborne disease transmission; the most evident environmental conditions are temperature and relative humidity. The sum of all the factors that influence temperature and humidity, either meteorological (outdoor) or human (indoor), as well as other circumstances influencing the spread of the droplets containing the infectious particles, as winds, or human behavior, sum up the factors influencing the transmission of airborne diseases.

- Climate and living area. Rainfall (number of rainy days being more important than total precipitation), mean of sunshine daily hours, latitude, altitude are characteristic agents to take in account when assessing the possibility of spread of any airborne infection. Furthermore, some infrequent or exceptional extreme events also influence the dissemination of airborne diseases, as tropical storms, hurricanes, typhoons, or monsoons. Climate conditions determine temperature, winds and relative humidity in any territory, either all year around or at isolated moments (days or weeks). Those are the main factors affecting the spread, duration and infectiousness of droplets containing infectious particles. For instance, influenza virus, is spread easily in northern countries (north hemisphere), because of climate conditions which favour the infectiousness of the virus but on the other hand, in those countries, lots of bacterial infections cannot spread outdoor most of the year, keeping in a latent stage.

- Socioeconomics and living conditions. They have a minor role in airborne diseases transmission, but they also have to be taken in consideration. Dwelling is an important aspect. In cities the spread of diseases is faster than in rural areas and outskirts. Normally, cities enclose quarters of buildings, in which the transmission of the viral and bacterial diseases among the neighborhoods is uncomplicated. However, suburban areas are generally more favourable for higher airborne fungal spores

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.

Outbreaks of zoonoses have been traced to human interaction with and exposure to animals at fairs, petting zoos, and other settings. In 2005, the Centers for Disease Control and Prevention (CDC) issued an updated list of recommendations for preventing zoonosis transmission in public settings. The recommendations, developed in conjunction with the National Association of State Public Health Veterinarians, include educational responsibilities of venue operators, limiting public and animal contact, and animal care and management.

Contact with farm animals can lead to disease in farmers or others that come into contact with infected animals. Glanders primarily affects those who work closely with horses and donkeys. Close contact with cattle can lead to cutaneous anthrax infection, whereas inhalation anthrax infection is more common for workers in slaughterhouses, tanneries and wool mills. Close contact with sheep who have recently given birth can lead to clamydiosis, or enzootic abortion, in pregnant women, as well as an increased risk of Q fever, toxoplasmosis, and listeriosis in pregnant or the otherwise immunocompromised. Echinococcosis is caused by a tapeworm which can be spread from infected sheep by food or water contaminated with feces or wool. Bird flu is common in chickens. While rare in humans, the main public health worry is that a strain of bird flu will recombine with a human flu virus and cause a pandemic like the 1918 Spanish flu. In 2017, free range chickens in the UK were temporarily ordered to remain inside due to the threat of bird flu. Cattle are an important reservoir of cryptosporidiosis and mainly affects the immunocompromised.

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".

Currently, no treatment is available.

Good husbandry measures, such as high water quality, low stocking density, and no mixing of batches, help to reduce disease incidence. To eradicate the disease, very strict protocol with regards to movement, water sources and stock replacement must be in place – and still it is difficult to achieve and comes at a high economic cost.

Caseous lymphadenitis (CLA) is an infectious disease caused by the bacterium "Corynebacterium pseudotuberculosis" found mostly in goats and sheep that at present has no cure. It manifests itself predominantly in the form of large, pus-filled cysts on the neck, sides and udders of goats and sheep. The disease is spread mostly from an animal coming in contact with pus from a burst cyst on an infected animal, but the disease is highly contagious and is thought to also be spread by coughing or even by flies. Studies have found CL incidence in commercial goat herds as high as 30%.

Fifth disease is transmitted primarily by respiratory secretions (saliva, mucus, etc.) but can also be spread by contact with infected blood. The incubation period (the time between the initial infection and the onset of symptoms) is usually between 4 and 21 days. Individuals with fifth disease are most infectious before the onset of symptoms. Typically, school children, day-care workers, teachers and parents are most likely to be exposed to the virus. When symptoms are evident, there is little risk of transmission; therefore, symptomatic individuals don't need to be isolated.

Symptoms of infectious mononucleosis are fever, sore throat, and swollen lymph glands. Sometimes, a swollen spleen or liver involvement may develop. Heart problems or involvement of the central nervous system occurs only rarely, and infectious mononucleosis is almost never fatal. There are no known associations between active EBV infection and problems during pregnancy, such as miscarriages or birth defects. Although the symptoms of infectious mononucleosis usually resolve in 1 or 2 months, EBV remains dormant or latent in a few cells in the throat and blood for the rest of the person's life. Periodically, the virus can reactivate and is commonly found in the saliva of infected persons. Reactivated and post-latent virus may pass the placental barrier in (also seropositive) pregnant women via macrophages and therefore can infect the fetus. Also re-infection of prior seropositive individuals may occur. In contrast, reactivation in adults usually occurs without symptoms of illness.

EBV also establishes a lifelong dormant infection in some cells of the body's immune system. A late event in a very few carriers of this virus is the emergence of Burkitt's lymphoma and nasopharyngeal carcinoma, two rare cancers. EBV appears to play an important role in these malignancies, but is probably not the sole cause of disease.

Most individuals exposed to people with infectious mononucleosis have previously been infected with EBV and are not at risk for infectious mononucleosis. In addition, transmission of EBV requires intimate contact with the saliva (found in the mouth) of an infected person. Transmission of this virus through the air or blood does not normally occur. The incubation period, or the time from infection to appearance of symptoms, ranges from 4 to 6 weeks. Persons with infectious mononucleosis may be able to spread the infection to others for a period of weeks. However, no special precautions or isolation procedures are recommended, since the virus is also found frequently in the saliva of healthy people. In fact, many healthy people can carry and spread the virus intermittently for life. These people are usually the primary reservoir for person-to-person transmission. For this reason, transmission of the virus is almost impossible to prevent.

The clinical diagnosis of infectious mononucleosis is suggested on the basis of the symptoms of fever, sore throat, swollen lymph glands, and the age of the patient. Usually, laboratory tests are needed for confirmation. Serologic results for persons with infectious mononucleosis include an elevated white blood cell count, an increased percentage of certain atypical white blood cells, and a positive reaction to a "mono spot" test.

Cat flu is the common name for a feline upper respiratory tract disease. While feline upper respiratory disease can be caused by several different pathogens, there are few symptoms that they have in common.

While Avian Flu can also infect cats, Cat flu is generally a misnomer, since it usually does not refer to an infection by an influenza virus. Instead, it is a syndrome, a term referring to the fact that patients display a number of symptoms that can be caused by one or more of the following infectious agents (pathogens):

1. Feline herpes virus causing feline viral rhinotracheitis (cat common cold, this is the disease that is closely similar to cat flu)

2. Feline calicivirus—(cat respiratory disease)

3. "Bordetella bronchiseptica"—(cat kennel cough)

4. "Chlamydophila felis"—(chlamydia)

In South Africa the term cat flu is also used to refer to Canine Parvo Virus. This is misleading, as transmission of the Canine Parvo Virus rarely involves cats.

Ultraviolet (UV) radiation is implicated in cattle with no pigmentation around the eyelids and cattle with prominently placed eyes. Exudate from the sun-burnt skin around the eyes can contain bacteria and attracts flies. UV light also directly damages the corneal epithelium, leading to a breakdown in host innate immunity.

Dust, dried-up plants, tall vegetation, and oversized or incorrectly placed ear tags may cause mechanical damage to the eye and facilitate bacterial colonization.

The disease may be complicated by concurrent infection with viruses such as infectious bovine rhinotracheitis virus (bovine herpesvirus 1) or adenovirus, bacteria such as "Mycoplasma boviculi" or "Listeria monocytogenes", or infestation by "Thelazia", a nematode.

Vitamin A deficiency is also implicated.

IBK is most prevalent in summer and early autumn.

A recent Meat and Livestock Australia report "estimates that the disease costs Australian beef producers AU$23.5 million annually in lost production and treatment costs".

"Moraxella bovis" is a Gram-negative rod-shaped aerobe. This bacterium is an obligate intracellular parasite of the mucous membranes, and can usually be isolated from the respiratory tract, vagina, and conjunctiva of healthy animals. Transmission of IBK is through direct contact with mucous membranes and their secretions and indirect contact where flies act as a mechanical vector. Asymptomatic carrier animals can also be source of infection.

White plague is a suite of coral diseases of which three types have been identified, initially in the Florida Keys. They are infectious diseases but it has proved difficult to identify the pathogens involved. White plague type II may be caused by the gram negative bacterium "Aurantimonas coralicida" in the order Rhizobiales but other bacteria have also been associated with diseased corals and viruses may also be implicated.

Disseminated disease refers to a diffuse disease-process, generally either infectious or neoplastic. The term may sometimes also characterize connective tissue disease.

A disseminated infection, for example, has extended beyond its origin or nidus and involved the bloodstream to "seed" other areas of the body. Similarly, one can view metastatic cancer as a disseminated infection in that it has extended into the bloodstream or into the lymphatic system and thus "seeded" distant sites (a process known as metastasis).

Disseminated disease often contrasts localized disease.

Most epidemics are caused by contagious diseases, with occasional exceptions, such as black plague. The spread of non-contagious communicable diseases, such as yellow fever or filariasis, is little or not affected by medical isolation (for ill persons) or medical quarantine (for exposed persons). Thus, a "contagious disease" is sometimes defined in practical terms, as a disease for which isolation or quarantine are useful public health responses.