Hemorrhagic smallpox is a severe form that is accompanied by extensive bleeding into the skin, mucous membranes, and gastrointestinal tract. This form develops in approximately 2 percent of infections and occurred mostly in adults. In hemorrhagic smallpox the skin does not blister, but remains smooth. Instead, bleeding occurs under the skin, making it look charred and black, hence this form of the disease is also known as black pox.

In the early, or fulminating form, hemorrhaging appears on the second or third day as sub-conjunctival bleeding turns the whites of the eyes deep red. Hemorrhagic smallpox also produces a dusky erythema, petechiae, and hemorrhages in the spleen, kidney, serosa, muscle, and, rarely, the epicardium, liver, testes, ovaries and bladder. Death often occurs suddenly between the fifth and seventh days of illness, when only a few insignificant skin lesions are present. A later form of the disease occurs in patients who survive for 8–10 days. The hemorrhages appear in the early eruptive period, and the rash is flat and does not progress beyond the vesicular stage. Patients in the early stage of disease show a decrease in coagulation factors (e.g. platelets, prothrombin, and globulin) and an increase in circulating antithrombin. Patients in the late stage have significant thrombocytopenia; deficiency of coagulation factors is less severe. Some in the late stage also show increased antithrombin. This form of smallpox occurs in anywhere from 3 to 25 percent of fatal cases depending on the virulence of the smallpox strain. Hemorrhagic smallpox is usually fatal.

In malignant-type smallpox (also called flat smallpox) the lesions remained almost flush with the skin at the time when raised vesicles form in the ordinary type. It is unknown why some people developed this type. Historically, it accounted for 5–10 percent of cases, and the majority (72 percent) were children. Malignant smallpox was accompanied by a severe prodromal phase that lasted 3–4 days, prolonged high fever, and severe symptoms of toxemia. The rash on the tongue and palate was extensive. Skin lesions matured slowly and by the seventh or eighth day they were flat and appeared to be buried in the skin. Unlike ordinary-type smallpox, the vesicles contained little fluid, were soft and velvety to the touch, and may have contained hemorrhages. Malignant smallpox was nearly always fatal.

Monkeypox is an infectious disease caused by the monkeypox virus. Symptoms begin with fever, headache, muscle pains, swollen lymph nodes, and feeling tired. This is then followed by a rash that forms blisters and crusts over. The time from exposure to onset of symptoms is around 10 days. The duration of symptoms is typically 2 to 5 weeks.

Monkeypox may be spread from handling bush meat, an animal bite or scratch, body fluids, contaminated objects, or close contact with an infected person. The virus is believed to normally circulate among certain rodents in Africa. Diagnosis can be confirmed by testing a lesion for the viruses DNA. The disease can appear similar to chickenpox.

The smallpox vaccine is believed to prevent infection. Cidofovir may be useful as treatment. The risk of death in those infected is up to 10%.

The disease mostly occurs in Central and West Africa. It was first identified in 1958 among laboratory monkeys. The first cases in humans were found in 1970 in the Democratic Republic of the Congo. An outbreak that occurred in the United States in 2003 was traced to a pet store where imported Gambian rodents were sold.

Monkeypox is similar to smallpox, although it is often milder.

Limited person-to-person spread of infection has been reported in disease-endemic areas in Africa. Case-fatality ratios in Africa have ranged from 1% to 10%.

Cowpox is an infectious disease caused by the cowpox virus. The virus, part of the orthopoxvirus family, is closely related to the "vaccinia" virus. The virus is zoonotic, meaning that it is transferable between species, such as from animal to human. The transferral of the disease was first observed in dairymaids who touched the udders of infected cows and consequently developed the signature pustules on their hands. Cowpox is more commonly found in animals other than bovines, such as rodents. Cowpox is similar to, but much milder than, the highly contagious and often deadly smallpox disease. Its close resemblance to the mild form of smallpox and the observation that dairymaids were immune from smallpox inspired the first smallpox vaccine, created and administered by English physician Edward Jenner.

The word “vaccination,” coined by Jenner in 1796, is derived from the Latin root "vaccinus", meaning of or from the cow. Once vaccinated, a patient develops antibodies that make them immune to cowpox, but they also develop immunity to the smallpox virus, or "Variola virus". The cowpox vaccinations and later incarnations proved so successful that in 1980, the World Health Organization announced that smallpox was the first disease to be eradicated by vaccination efforts worldwide. Other orthopox viruses remain prevalent in certain communities and continue to infect humans, such as the cowpox virus (CPXV) in Europe, vaccinia in Brazil, and monkeypox virus in Central and West Africa.

Paravaccinia virus presents itself with blisters, nodules, or lesions about 4 mm in diameter, typically in the area that has made contact with livestock that is infected with bovine papular stomatitis. Lesions may begin forming as late as three weeks after contact has been made with an infected animal. In rare cases, lesions may be seen systemic. General signs of infection are also common, such as fever and fatigue.

Infected livestock may present with blisters or lesions on their udders or snout. Often, however, infected livestock show little to no symptoms.

The early (prodromal) symptoms in adolescents and adults are nausea, loss of appetite, aching muscles, and headache. This is followed by the characteristic rash or oral sores, malaise, and a low-grade fever that signal the presence of the disease. Oral manifestations of the disease (enanthem) not uncommonly may precede the external rash (exanthem). In children the illness is not usually preceded by prodromal symptoms, and the first sign is the rash or the spots in the oral cavity. The rash begins as small red dots on the face, scalp, torso, upper arms and legs; progressing over 10–12 hours to small bumps, blisters and pustules; followed by umbilication and the formation of scabs.

At the blister stage, intense itching is usually present. Blisters may also occur on the palms, soles, and genital area. Commonly, visible evidence of the disease develops in the oral cavity and tonsil areas in the form of small ulcers which can be painful or itchy or both; this enanthem (internal rash) can precede the exanthem (external rash) by 1 to 3 days or can be concurrent. These symptoms of chickenpox appear 10 to 21 days after exposure to a contagious person. Adults may have a more widespread rash and longer fever, and they are more likely to experience complications, such as varicella pneumonia.

Because watery nasal discharge containing live virus usually precedes both exanthem (external rash) and enanthem (oral ulcers) by 1 to 2 days, the infected person actually becomes contagious one to two days before recognition of the disease. Contagiousness persists until all vesicular lesions have become dry crusts (scabs), which usually entails four or five days, by which time nasal shedding of live virus ceases.

The condition usually resolves by itself within a couple of weeks. The rash may, however, last for up to one month. Chickenpox is contagious starting from one to two days before the appearance of the rash and lasts until the lesions have crusted.

Chickenpox is rarely fatal, although it is generally more severe in adult men than in women or children. Non-immune pregnant women and those with a suppressed immune system are at highest risk of serious complications. Arterial ischemic stroke (AIS) associated with chickenpox in the previous year accounts for nearly one third of childhood AIS. The most common late complication of chickenpox is shingles (herpes zoster), caused by reactivation of the "varicella zoster" virus decades after the initial, often childhood, chickenpox infection.

Chickenpox, also known as varicella, is a highly contagious disease caused by the initial infection with varicella zoster virus (VZV). The disease results in a characteristic skin rash that forms small, itchy blisters, which eventually scab over. It usually starts on the chest, back, and face then spreads to the rest of the body. Other symptoms may include fever, tiredness, and headaches. Symptoms usually last five to seven days. Complications may occasionally include pneumonia, inflammation of the brain, and bacterial skin infections. The disease is often more severe in adults than in children. Symptoms begin 10 to 21 days after exposure to the virus.

Chickenpox is an airborne disease which spreads easily through the coughs and sneezes of an infected person. It may be spread from one to two days before the rash appears until all lesions have crusted over. It may also spread through contact with the blisters. Those with shingles may spread chickenpox to those who are not immune through contact with the blisters. The disease can usually be diagnosed based on the presenting symptom; however, in unusual cases it may be confirmed by polymerase chain reaction (PCR) testing of the blister fluid or scabs. Testing for antibodies may be done to determine if a person is or is not immune. People usually only get chickenpox once. Although reinfections by the virus occur, these reinfections usually do not cause any symptoms.

The varicella vaccine has resulted in a decrease in the number of cases and complications from the disease. It protects about 70 to 90 percent of people from disease with a greater benefit for severe disease. Routine immunization of children is recommended in many countries. Immunization within three days of exposure may improve outcomes in children. Treatment of those infected may include calamine lotion to help with itching, keeping the fingernails short to decrease injury from scratching, and the use of paracetamol (acetaminophen) to help with fevers. For those at increased risk of complications antiviral medication such as aciclovir are recommended.

Chickenpox occurs in all parts of the world. In 2013 there were 140 million cases of chickenpox and herpes zoster worldwide. Before routine immunization the number of cases occurring each year was similar to the number of people born. Since immunization the number of infections in the United States has decreased nearly 90%. In 2015 chickenpox resulted in 6,400 deaths globally – down from 8,900 in 1990. Death occurs in about 1 per 60,000 cases. Chickenpox was not separated from smallpox until the late 19th century. In 1888 its connection to shingles was determined. The first documented use of the term "chicken pox" was in 1658. Various explanations have been suggested for the use of "chicken" in the name, one being the relative mildness of the disease.

Pigeon pox is a viral disease to which pigeons are susceptible. There is a live viral vaccine available (ATCvet code: ). Pigeon pox is caused by a virus that is spread by mosquitoes and dirty water but not in droppings.

Paravaccinia virus is a viral infection of the Parapoxvirus genus of viruses. Human can contract the virus from contact with livestock infected with Bovine papular stomatitis and is common with ranchers, milkers, and veterinarians. Infection will present with fever, fatigue, and lesion on the skin.

Variola caprina (goat pox) is a contagious viral disease caused by a pox virus that affects goats. The virus usually spreads via the respiratory system, and sometimes spreads through abraded skin. It is most likely to occur in crowded stock. Sources of the virus include cutaneous lesions, saliva, nasal secretions and faeces. There are two types of the disease: the papulo-vesicular form and the nodular form (stone pox). The incubation period is usually 8–13 days, but it may be as short as four days.

It is thought the same virus spreads sheep pox, to which European sheep breeds are highly susceptible. The virus may be present in dried scabs for up to six months.

In endemic areas the morbidity rate is 70–90% and the mortality rate is 5–10%. The mortality rate may reach nearly 100% in imported animals. Resistant animals may show only a mild form of the disease, which may be missed as only a few lesions are present, usually around the ears or the tail.

Alastrim, also known as variola minor, was the milder strain of the variola virus that caused smallpox. The last known case of variola minor was in Somalia, Africa in 1977. Smallpox was formally declared eradicated in May 1980.

Variola minor is of the genus orthopoxvirus, which are DNA viruses that replicate in the cytoplasm of the affected cell, rather than in its nucleus. Like variola major, alastrim was spread through inhalation of the virus in the air, which could occur through face-to-face contact or through fomites. Infection with variola minor conferred immunity against the more dangerous variola major.

Variola minor was a less common form of the virus, and much less deadly. Although alastrim had the same incubation period and pathogenetic stages as smallpox, alastrim is believed to have had a mortality rate of less than 1%, as compared to smallpox's 30%.

Because alastrim was a less debilitating disease than smallpox, patients were more frequently ambulant and thus able to infect others more rapidly. As such, variola minor swept through the USA, Great Britain, and South Africa in the early 20th century, becoming the dominant form of the disease in those areas and thus rapidly decreasing mortality rates.

Alastrim was also called white pox, kaffir pox, Cuban itch, West Indian pox, milk pox, and pseudovariola.

Like smallpox, alastrim has now been totally eradicated from the globe thanks to the 1960s Global Smallpox Eradication campaign. The last case of indigenous variola minor was reported in a Somalian cook, Ali Maow Maalin, in October 1977, and smallpox was officially declared eradicated worldwide in May 1980.

Hemorrhagic smallpox, sometimes called bloody pox, fulminant smallpox, and blackpox, is a severe and rare form of smallpox and is usually fatal. Like all forms of smallpox it is caused by the variola virus. It is characterized by an incubation period of 7 to 14 days. It has two stages, the first begins with fever, headache, chills, nausea, vomiting and severe muscle aches. The skin flushes in a deep-purple, uneven pattern across the face. The early stage is often mistaken for measles. The late stage is characterized by the appearance of small blisters resembling a severe form of chickenpox. These small blisters then flatten until they are even with the skin, and change into reddish lesions similar to those seen in measles. The skin then turns a deep purple. Lesions appear inside the mouth and active bleeding from oral and nasal mucous membranes is common. This is followed by active bleeding in the gastrointestinal tract, and blood appears in the stool and urine. Blood studies resemble the clinical values of disseminated intravascular coagulation.

Herpes gladiatorum is characterized by a rash with clusters of sometimes painful fluid-filled blisters, often on the neck, chest, face, stomach, and legs. The infection is often accompanied by lymphadenopathy (enlargement of the lymph nodes), fever, sore throat, and headache. Often, the accompanying symptoms are much more of an inconvenience than the actual skin blisters and rash.

Each blister contains infectious virus particles (virions). Close contact, particularly abrasive contact as found in contact sports, causes the infected blisters to burst and pass the infection along. Autoinoculation (self-infection) can occur through self-contact, leading to infection at multiple sites on the body.

Herpes gladiatorum symptoms may last up to a few weeks, and if they occur during the first outbreak, they can be more pronounced. In recurrences of the ailment, symptoms are milder, even if lesions still tend to occur. With recurrent infections scabs may form at 3 days yet the lesions are still considered infectious up til 6.4 days after starting oral antiviral medications. Healing takes place without leaving scars. It is possible that the condition evolves asymptomatically and sores are never present.

Herpes gladiatorum is one of the most infectious of herpes-caused diseases, and is transmissible by skin-to-skin contact. The disease was first described in the 1960s in the "New England Journal of Medicine". It is caused by contagious infection with human herpes simplex virus type 1 (HSV-1), which more commonly causes oral herpes (cold sores). Another strain, HSV-2 usually causes genital herpes, although the strains are very similar and either can cause herpes in any location.

While the disease is commonly passed through normal human contact, it is strongly associated with contact sports—outbreaks in sporting clubs being relatively common.

Other names for the disease are herpes rugbiorum or "scrumpox" (after rugby football), "wrestler's herpes" or "mat pox" (after wrestling). In one of the largest outbreaks ever among high-school wrestlers at a four-week intensive training camp, HSV was identified in 60 of 175 wrestlers. Lesions were on the head in 73 percent of the wrestlers, the extremities in 42 percent, and the trunk in 28 percent. Physical symptoms sometimes recur in the skin. Previous adolescent HSV-1 seroconversion would preclude most herpes gladiatorum, but being that stress and trauma are recognized triggers, such a person would be likely to infect others.

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.

Eczema vaccinatum is a rare severe adverse reaction to smallpox vaccination.

It is characterized by serious local or disseminated, umbilicated, vesicular, crusting skin rashes in the face, neck, chest, abdomen, upper limbs and hands, caused by widespread infection of the skin in people with previous diagnosed skin conditions such as eczema or atopic dermatitis, even if the conditions are not active at the time. Other signs and symptoms include fever and facial and supraglottic edema. The condition may be fatal if severe and left untreated. Survivors are likely to have some scarring (pockmarks).

Smallpox vaccine should not be given to patients with a history of eczema. Because of the danger of transmission of vaccinia, it also should not be given to people in close contact with anyone who has active eczema and who has not been vaccinated. People with other skin diseases (such as atopic dermatitis, burns, impetigo, or herpes zoster) also have an increased risk of contracting eczema vaccinatum and should not be vaccinated against smallpox.

Eczema is also associated with increased complications related to other vesiculating viruses such as chickenpox; this is called eczema herpeticum.

A culture of vesicular fluid will grow vaccinia virus. Skin biopsy shows necrotic epidermal cells with intranuclear inclusions.

Cowpox originates on the udders or teats of cows. It is classified as a zoonotic disease, which means it can be transferred from animals to humans and vice versa. Cowpox is an infectious disease. So, the disease can manifest on cows in environments where bacteria thrive, due to unsanitary conditions, or randomly. Cowpox symptoms are similar in whichever host they infect: cow, cat, human. Cowpox symptoms include round, pus filled lesions on the skin at the site of infection. In most cases of humans, the lesions develop on the inner and outer parts of the hand and fingers. In some cases, the infected person can develop a mild fever or inflammation around the lesions. Cowpox can be transferred from human to human by contact of the infected site to another individual. It is very similar in pathology and structure in contrast to small pox. However, cowpox has increased activity in between the ectoderm and endoderm layers of the human skin. Cowpox includes both A type bodies and B type inclusion bodies which largely impacts the pathology of the disease.

Farmyard pox is a group of closely related parapoxviruses of sheep and cattle that cause similar diseases in humans. Conditions included in this group are:

External abscesses are the most common form of pigeon fever seen in horses. Abscesses develop on the body, usually in the pectoral region and along the ventral midline of the abdomen. However, abscesses can also develop on other areas of the body such as the prepuce, mammary gland, triceps, limbs and head. The fatality rate for this form infection is very low. The abscess is often drained once it has matured.

This is the least common form of pigeon fever seen in horses. It is characterized by severe limb swelling and cellulitis in one or both hind limbs and can lead to lameness, fever, lethargy and loss of appetite. Antimicrobial and anti-inflammatory treatments are required to prevent further complications such as limb edema, prolonged or recurrent infection, lameness, weakness and weight loss.

Goat pox is found in the part of Africa north of the equator, the Middle East, Central Asia and India. It may be spread between animals by:

- Direct contact

- Indirect transmission by contaminated implements, vehicles or products such as litter or fodder

- Indirect transmission by insects (mechanical vectors).

- Contamination by inhalation, intradermal or subcutaneous inoculation, or by respiratory, transcutaneous and transmucosal routes

Secondary syphilis occurs approximately four to ten weeks after the primary infection. While secondary disease is known for the many different ways it can manifest, symptoms most commonly involve the skin, mucous membranes, and lymph nodes. There may be a symmetrical, reddish-pink, non-itchy rash on the trunk and extremities, including the palms and soles. The rash may become maculopapular or pustular. It may form flat, broad, whitish, wart-like lesions known as condyloma latum on mucous membranes. All of these lesions harbor bacteria and are infectious. Other symptoms may include fever, sore throat, malaise, weight loss, hair loss, and headache. Rare manifestations include liver inflammation, kidney disease, joint inflammation, periostitis, inflammation of the optic nerve, uveitis, and interstitial keratitis. The acute symptoms usually resolve after three to six weeks; about 25% of people may present with a recurrence of secondary symptoms. Many people who present with secondary syphilis (40–85% of women, 20–65% of men) do not report previously having had the classic chancre of primary syphilis.

Latent syphilis is defined as having serologic proof of infection without symptoms of disease. It is further described as either early (less than 1 year after secondary syphilis) or late (more than 1 year after secondary syphilis) in the United States. The United Kingdom uses a cut-off of two years for early and late latent syphilis. Early latent syphilis may have a relapse of symptoms. Late latent syphilis is asymptomatic, and not as contagious as early latent syphilis.