In humans, "Erysipelothrix rhusiopathiae" infections most commonly present in a mild cutaneous form known as erysipeloid or fish poisoning. "E. rhusiopathiae" can cause an indolent cellulitis, more commonly in individuals who handle fish and raw meat. It gains entry typically by abrasions in the hand. Bacteremia and endocarditis are uncommon but serious sequelae. Due to the rarity of reported human cases, "E. rhusiopathiae" infections are frequently misidentified at presentation.

The treatment of choice is a single dose of benzathine benzylpenicillin given by intramuscular injection, or a five-day to one-week course of either oral penicillin or intramuscular procaine benzylpenicillin. Erythromycin or doxycycline may be given instead to people who are allergic to penicillin. "E. rhusiopathiae" is intrinsically resistant to vancomycin.

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.

Infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

Infections are caused by infectious agents including viruses, viroids, prions, bacteria, nematodes such as parasitic roundworms and pinworms, arthropods such as ticks, mites, fleas, and lice, fungi such as ringworm, and other macroparasites such as tapeworms and other helminths.

Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response.

Specific medications used to treat infections include antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics. Infectious diseases resulted in 9.2 million deaths in 2013 (about 17% of all deaths). The branch of medicine that focuses on infections is referred to as infectious disease.

A sharp rise in mortality is often seen (depending on the virulence of the disease). Other clinical signs include abdominal swelling, anorexia, abnormal swimming, darkening of the skin, and trailing of the feces from the vent. On necropsy, internal damage (viral necrosis) to the pancreas and thick mucus in the intestines often is present. Surviving fish should recover within one to two weeks.

Diagnostic methods for the detection of the disease include: characteristic histological pancreatic lesion, PCR, indirect fluorescent antibody testing, ELISA, and virus culture. High virus titers can be isolated from carrier animals.

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.

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.

A skin and skin structure infection (SSSI), also referred to as skin and soft tissue infection (SSTI) or acute bacterial skin and skin structure infection (ABSSSI), is an infection of skin and associated soft tissues (such as loose connective tissue and mucous membranes). The pathogen involved is usually a bacterial species. Such infections often requires treatment by antibiotics.

Until 2008, two types were recognized, complicated skin and skin structure infection (cSSSI) and uncomplicated skin and skin structure infection (uSSSI). "Uncomplicated" SSSIs included simple abscesses, impetiginous lesions, furuncles, and cellulitis. "Complicated" SSSIs included infections either involving deeper soft tissue or requiring significant surgical intervention, such as infected ulcers, burns, and major abscesses or a significant underlying disease state that complicates the response to treatment. Superficial infections or abscesses in an anatomical site, such as the rectal area, where the risk of anaerobic or gram-negative pathogen involvement is higher, should be considered complicated infections. The two categories had different regulatory approval requirements. The uncomplicated category (uSSSI) is normally only caused by "Staphylococcus aureus" and "Streptococcus pyogenes", whereas the complicated category (cSSSI) might also be caused by a number of other pathogens. In cSSSI, the pathogen is known in only about 40% of cases.

Because cSSSIs are usually serious infections, physicians do not have the time for a culture to identify the pathogen, so most cases are treated empirically, by choosing an antibiotic agent based on symptoms and seeing if it works. For less severe infections, microbiologic evaluation via tissue culture has been demonstrated to have high utility in guiding management decisions. To achieve efficacy, physicians use broad-spectrum antibiotics. This practice contributes in part to the growing incidence of antibiotic resistance, a trend exacerbated by the widespread use of antibiotics in medicine in general. The increased prevalence of antibiotic resistance is most evident in methicillin-resistant "Staphylococcus aureus" (MRSA). This species is commonly involved in cSSSIs, worsening their prognosis, and limiting the treatments available to physicians. Drug development in infectious disease seeks to produce new agents that can treat MRSA.

Since 2008, the U.S. Food and Drug Administration has changed the terminology to "acute bacterial skin and skin structure infections" (ABSSSI). The Infectious Diseases Society of America (IDSA) has retained the term "skin and soft tissue 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.

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%.

Flacherie (literally: "flaccidness") is a disease of silkworms, caused by silkworms eating infected or contaminated mulberry leaves. Flacherie infected silkworms look weak and can die from this disease. Silkworm larvae that are about to die from Flacherie are a dark brown.

There are two kinds of flacherie: essentially, infectious (viral) flacherie and noninfectious ("bouffee") flacherie. Both are technically a lethal diarrhea.

Bouffée flacherie is caused by heat waves ("bouffée" means "sudden heat spell" in French).

Viral flacherie is ultimately caused by infection with "Bombyx mori" infectious flacherie virus (BmIFV, Iflaviridae), "Bombyx mori" densovirus (BmDNV, Parvoviridae) or "Bombyx mori" cypovirus 1 (BmCPV-1, Reoviridae). This either alone or in combination with bacterial infection destroys the gut tissue. Bacterial pathogens contributing to infectious flaccherie are "Serratia marcescens", and species of "Streptococcus" and "Staphylococcus" in the form known as thatte roga.

Louis Pasteur, who began his studies on silkworm diseases in 1865, was the first one able to recognize that mortality due to viral flacherie was caused by infection. (Priority, however, was claimed by Antoine Béchamp.) Richard Gordon described the discovery: "The French silk industry was meanwhile plummeting from a 130 million to an 8 million francs annual income, because the silkworms had all caught "pébrine," black pepper disease…He [Pasteur] went south from Paris to Alais, and rewarded them by discovering the silkworm epidemic to be inflicted by some sort of living microbe…Pasteur threw in another disease, "flâcherie," silkworm diarrhoea. The cures for both were culling the insects which showed the peppery spots — the peasants bottled the silkworm moths in brandy, for display to the experts — and rigorous hygiene of the mulberry leaf."

A contagious disease is a subset category of transmissible diseases, which are transmitted to other persons, either by physical contact with the person suffering the disease, or by casual contact with their secretions or objects touched by them or airborne route among other routes.

Non-contagious infections, by contrast, usually require a special mode of transmission between persons or hosts. These include need for intermediate vector species (mosquitoes that carry malaria) or by non-casual transfer of bodily fluid (such as transfusions, needle sharing or sexual contact).

The boundary between contagious and non-contagious infectious diseases is not perfectly drawn, as illustrated classically by tuberculosis, which is clearly transmissible from person to person, but was not classically considered a contagious disease. In the present day, most sexually transmitted diseases are considered contagious, but only some of them are subject to medical isolation.

Acanthocheilonemiasis is a rare tropical infectious disease caused by a parasite known as "Acanthocheilonema perstans". It can cause skin rashes, abdominal and chest pains, muscle and joint pains, neurological disorders and skin lumps. It is mainly found in Africa. The parasite is transmitted through the bite of small flies. Studies show that there are elevated levels of white blood cells.

Acanthocheilonemiasis belongs to a group of parasitic diseases known as filarial disease (nematode), all of which are classified as Neglected Tropical Diseases. Filarial disease results when microfilariae, which are nematode larvae, reach the lymphatic system; microfilariae reside in the serous cavities of humans. They have a five-stage life cycle that includes birth to thousands of live microfilariae within the host (i.e. human body), and then translocation via blood meal to the dermis layer of the skin. It is here that microfilariae cause major symptoms, which are edema and thickening of the skin and underlying connective tissues. It can also cause skin rashes, abdominal and chest pains, muscle (myalgia) and joint pains, neurological disorders and skin lumps. In addition, it causes spleen and liver enlargement, which is called hepatosplenomegaly. Studies show elevated levels of leukocytes, or white blood cells, which is referred to as eosinophilia. It is mainly found in Africa. The parasite is transmitted through the bite of small flies ("A. coliroides").

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.

Generally speaking, acanthocheilonemiasis does not show initial symptoms. However, if symptoms do arise, it is typically in individuals who are visiting highly infected areas rather than natives to those areas. A major common laboratory finding is an increase in specialized white blood cells, which is called eosinophilia.

Other symptoms include itchy skin, neurological symptoms, abdominal and chest pain, muscle pain, and swelling underneath the skin. If there are abnormally high levels of white blood cells, then a physical examination will most likely find an enlarged spleen or liver.

In certain scenarios, nematodes may physically lodge into the chest or abdomen, resulting in an inflammation. Diagnosis of this condition usually occurs via a blood smear examination under light microscopy.

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".

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".

Originally, the term referred as sometimes been broadened to encompass "any" communicable or infectious disease. Often the word can only be understood in context, where it is used to emphasise very infectious, easily transmitted, or especially severe communicable disease. They could be very dangerous.

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.

Affected individuals typically develop symptoms including high fevers, shaking, chills, fatigue, headaches, vomiting, and general illness within 48 hours of the initial infection. The erythematous skin lesion enlarges rapidly and has a sharply demarcated, raised edge. It appears as a red, swollen, warm, and painful rash, similar in consistency to an orange peel. More severe infections can result in vesicles (pox or insect bite-like marks), blisters, and petechiae (small purple or red spots), with possible skin necrosis (death). Lymph nodes may be swollen, and lymphedema may occur. Occasionally, a red streak extending to the lymph node can be seen.

The infection may occur on any part of the skin, including the face, arms, fingers, legs, and toes; it tends to favour the extremities. Fat tissue and facial areas, typically around the eyes, ears, and cheeks, are most susceptible to infection. Repeated infection of the extremities can lead to chronic swelling (lymphangitis).

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

Airborne diseases include any that are caused via transmission through the air. Many airborne diseases are of great medical importance. The pathogens transmitted may be any kind of microbe, and they may be spread in aerosols, dust or liquids. The aerosols might be generated from sources of infection such as the bodily secretions of an infected animal or person, or biological wastes such as accumulate in lofts, caves, garbage and the like. Such infected aerosols may stay suspended in air currents long enough to travel for considerable distances, though the rate of infection decreases sharply with the distance between the source and the organism infected.

Airborne pathogens or allergens often cause inflammation in the nose, throat, sinuses and the lungs. This is caused by the inhalation of these pathogens that affect a person's respiratory system or even the rest of the body. Sinus congestion, coughing and sore throats are examples of inflammation of the upper respiratory air way due to these airborne agents. Air pollution plays a significant role in airborne diseases which is linked to asthma. Pollutants are said to influence lung function by increasing air way inflammation.

Many common infections can spread by airborne transmission at least in some cases, including: Anthrax (inhalational), Chickenpox, Influenza, Measles, Smallpox, Cryptococcosis, and Tuberculosis.

Airborne diseases can also affect non-humans. For example, Newcastle disease is an avian disease that affects many types of domestic poultry worldwide which is transmitted via airborne contamination.

Often, airborne pathogens or allergens cause inflammation in the nose, throat, sinuses, and the upper airway lungs. Upper airway inflammation causes coughing congestion, and sore throat. This is caused by the inhalation of these pathogens that affect a person's respiratory system or even the rest of the body. Sinus congestion, coughing and sore throats are examples of inflammation of the upper respiratory air way due to these airborne agents.

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.

Erysipelas is an acute infection typically with a skin rash, usually on any of the legs and toes, face, arms, and fingers. It is an infection of the upper dermis and superficial lymphatics, usually caused by beta-hemolytic group A "Streptococcus" bacteria on scratches or otherwise infected areas. Erysipelas is more superficial than cellulitis, and is typically more raised and demarcated. The term is from Greek ἐρυσίπελας, meaning "red skin".