The types of neurosyphilis include asymptomatic, acute syphilitic meningitis, meningovascular syphilis, parenchymatous syphilis (which includes general paresis and tabes dorsalis), and optic atrophy.

Treponematosis is a term used to collectively or individually describe any of the diseases caused by the bacterial species "Treponema". There are four subspecies described which cause the following diseases:

- Syphilis ("Treponema pallidum pallidum")

- Yaws ("Treponema pallidum pertenue")

- Bejel ("Treponema pallidum endemicum")

- Pinta ("Treponema carateum")

Bejel usually begins in childhood as a small mucous patch, often on the interior of the mouth, followed by the appearance of raised, eroding lesions on the limbs and trunk. Periostitis (inflammation) of the leg bones is commonly seen, and gummas of the nose and soft palate develop in later stages.

The secondary stages of syphilis persists to be more dangerous to the systems of the human body. The disseminated disease can cause constitutional symptoms and condylomata lata. Many treponemes are present in chancres in the primary stage; however, condylomata lata is usually present in the secondary stage. The pathogen can spread through blood, which can infect the vessels in the body. The infection of the heart, muscles, and vessels in the body can lead to meningovascular syphilis. Generally, rashes may start developing on the hands and soles of the feet, and it can spread to various parts of skin on the body. Other symptoms may include sore throat, headache, joint pain, fever, and patches of hair loss. As in stage one, lesions may start to form on the body, but in this stage in particular, lesions are found in mucous membranes of the mouth, throat, bones, and internal organs. Also common with stage one, the symptoms and signs of secondary syphilis will go away with or without treatment and medication. The diagnosis includes serology nonspecific and specific, both positive. The secondary stage is however highly infectious because the bacteria is spreading drastically throughout the body.

Although the organism that causes bejel, "Treponema pallidum endemicum", is morphologically and serologically indistinguishable from "Treponema pallidum pallidum", which causes venereal syphilis, transmission of bejel is not venereal in nature, generally resulting from mouth-to-mouth contact or sharing of domestic utensils, and the courses of the two diseases are somewhat different.

Within 90 days (but usually less than a month) of infection a painless but distinctive "mother yaw" nodule appears, which enlarges and becomes warty in appearance. Nearby "daughter yaws" may also appear simultaneously.

This primary stage resolves completely within six months. The secondary stage occurs months to years later, with typically widespread skin lesions that vary in appearance, including "crab yaws" on the palms of the hands and soles of the feet with desquamation. These secondary lesions frequently ulcerate and are then highly infectious, but heal after six months or more. About 10% of people then go on to develop tertiary disease within five to ten years (during which further secondary lesions may come and go), with widespread bone, joint and soft tissue destruction, which may include extensive destruction of the bone and cartilage of the nose (Rhinopharyngitis mutilans or "gangosa").

Pinta, the least severe of treponemal infections being limited to the skin, is thought to be transmitted by skin-to-skin contact (similar to bejel and yaws), and after an incubation period of two to three weeks, produces a raised papule, which enlarges and becomes hyperkeratotic (scaly/flaky). Lesions are usually present in the exposed surface of arms and legs. Local lymph nodes might be enlarged. Three to 9 months later, further thickened and flat lesions (pintids) appear all over the body. These generally resolve, but a proportion of people with pinta will go on to develop late-stage disease, characterised by widespread pigmentary change with a mixture of hyperpigmentation and depigmentation which can be disfiguring.

Yaws is a tropical infection of the skin, bones and joints caused by the spirochete bacterium "Treponema pallidum pertenue". The disease begins with a round, hard swelling of the skin, 2 to 5 centimeters in diameter. The center may break open and form an ulcer. This initial skin lesion typically heals after three to six months. After weeks to years, joints and bones may become painful, fatigue may develop, and new skin lesions may appear. The skin of the palms of the hands and the soles of the feet may become thick and break open. The bones (especially those of the nose) may become misshapen. After five years or more large areas of skin may die, leaving a scar.

Yaws is spread by direct contact with the fluid from a lesion of an infected person. The contact is usually of a non-sexual nature. The disease is most common among children, who spread it by playing together. Other related treponemal diseases are bejel ("Treponema pallidum endemicum"), pinta ("Treponema pallidum carateum"), and syphilis ("Treponema pallidum pallidum"). Yaws is often diagnosed by the appearance of the lesions. Blood antibody tests may be useful but cannot separate previous from current infections. Polymerase chain reaction (PCR) is the most accurate method of diagnosis.

Prevention is, in part, by curing those who have the disease thereby decreasing the risk of transmission. Where the disease is common, treating the entire community is effective. Improving cleanliness and sanitation will also decrease spread. Treatment is typically with antibiotics including: azithromycin by mouth or benzathine penicillin by injection. Without treatment, physical deformities occur in 10% of cases.

Yaws is common in at least 14 tropical countries as of 2012. The disease only infects humans. In the 1950s and 1960s the World Health Organization (WHO) nearly eradicated yaws. Since then the number of cases has increased and there are renewed efforts to globally eradicate the disease by 2020. The last estimate of the number of people infected was more than 500,000 in 1995. Although one of the first descriptions of the disease was made in 1679 by Willem Piso, archaeological evidence suggests that yaws may have been present among humans as far back as 1.6 million years ago.

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.

Primary syphilis is typically acquired by direct sexual contact with the infectious lesions of another person. Approximately 3 to 90 days after the initial exposure (average 21 days) a skin lesion, called a chancre, appears at the point of contact. This is classically (40% of the time) a single, firm, painless, non-itchy skin ulceration with a clean base and sharp borders 0.3–3.0 cm in size. The lesion may take on almost any form. In the classic form, it evolves from a macule to a papule and finally to an erosion or ulcer. Occasionally, multiple lesions may be present (~40%), with multiple lesions more common when coinfected with HIV. Lesions may be painful or tender (30%), and they may occur in places other than the genitals (2–7%). The most common location in women is the cervix (44%), the penis in heterosexual men (99%), and anally and rectally relatively commonly in men who have sex with men (34%). Lymph node enlargement frequently (80%) occurs around the area of infection, occurring seven to 10 days after chancre formation. The lesion may persist for three to six weeks without treatment.

Diagnosis is usually clinical, but as with yaws and bejel, serological tests for syphilis, such as rapid plasma reagin (RPR) and TPHA, will be positive, and the spirochetes can be seen on dark field microscopy of samples taken from the early papules.

Differences between the conditions chancre and chancroid:

- Chancre is a lesion typical of infection with the bacterium that causes syphilis, Treponema pallidum

- Chancroid is a lesion typical of infection with the bacterium Haemophilus ducreyi

- Chancres are typically painless, whereas chancroid are typically painful

- Chancres are typically single, whereas chancroid are typically multiple

- Chancres cause regional bilateral lymph node enlargement, whereas chancroid cause regional unilateral lymph node enlargement

- Chancres typically exude serum, whereas chancroid typically have a grey or yellow purulent exudate

- Chancres have a hard (indurated) base with sloping edges, whereas chancroid have a soft base with undermined edges

- Chancres heal spontaneously within three to six weeks, even in the absence of treatment

- Chancres can occur in the pharynx as well as on the genitals. Not to be confused with condyloma lata, which is seen in secondary syphilis

A subclinical infection (sometimes called a preinfection) is an infection that, being , is nearly or completely asymptomatic (no signs or symptoms). A subclinically infected person is thus an asymptomatic carrier of a microbe, intestinal parasite, or virus that usually is a pathogen causing illness, at least in some individuals. Many pathogens spread by being silently carried in this way by some of their host population. Such infections occur both in humans and nonhuman animals. An example of an asymptomatic infection is a mild common cold that is not noticed by the infected individual. Since subclinical infections often occur without eventual overt sign, their existence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction.

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.

These are only local and no systemic manifestations are present.

The ulcer characteristically:

- Ranges in size dramatically from 3 to 50 mm (1/8 inch to two inches) across

- Is painful

- Has sharply defined, undermined borders

- Has irregular or ragged borders

- Has a base that is covered with a gray or yellowish-gray material

- Has a base that bleeds easily if traumatized or scraped

- painful swollen lymph nodes occurs in 30 to 60% of patients.

- dysuria (pain with urination) and dyspareunia (pain with intercourse) in females

About half of infected men have only a single ulcer. Women frequently have four or more ulcers, with fewer symptoms.

The initial ulcer may be mistaken as a "hard" chancre, the typical sore of primary syphilis, as opposed to the "soft chancre" of chancroid.

Approximately one-third of the infected individuals will develop enlargements of the inguinal lymph nodes, the nodes located in the fold between the leg and the lower abdomen.

Half of those who develop swelling of the inguinal lymph nodes will progress to a point where the nodes rupture through the skin, producing draining abscesses. The swollen lymph nodes and abscesses are often referred to as buboes.

The signs and symptoms of a vertically transmitted infection depend on the individual pathogen. It may cause subtle signs such as a influenza-like illness and may not even be noticed by the mother during the pregnancy. In such cases, the effects may be seen first at birth.

Symptoms of a vertically transmitted infection may include fever and flu like symptoms. The newborn is often small for gestational age. A petechial rash on the skin may be present, with small reddish or purplish spots due to bleeding from capillaries under the skin. An enlarged liver and spleen (hepatosplenomegaly) is common, as is jaundice. However, jaundice is less common in hepatitis B because a newborn's immune system is not developed well enough to mount a response against liver cells, as would normally be the cause of jaundice in an older child or adult. Hearing impairment, eye problems, mental retardation, autism, and death can be caused by vertically transmitted infections. The mother often has a mild infection with few or no symptoms.

The genetic conditions of Aicardi-Goutieres syndrome are possibly present in a similar manner.

Feline zoonosis are the viral, bacterial, fungal, protozoan, nematode and arthropod infections that can be transmitted to humans from the domesticated cat, "Felis catus". Some of these are diseases are reemerging and newly emerging infections or infestations caused by zoonotic pathogens transmitted by cats. In some instances, the cat can display symptoms of infection (these may differ from the symptoms in humans) and sometimes the cat remains asymptomatic. There can be serious illnesses and clinical manifestations in people who become infected. This is dependent on the immune status and age of the person. Those who live in close association with cats are more prone to these infections. But those that do not keep cats as pets are also able to acquire these infections because of the transmission can be from cat feces and the parasites that leave their bodies.

People can acquire cat-associated infections through bites, scratches or other direct contact of the skin or mucous membranes with the cat. This includes 'kissing' or letting the animal lick the mouth or nose. Mucous membranes are easily infected when the pathogen is in the mouth of the cat. Pathogens can also infect people when there is contact with animal saliva, urine and other body fluids or secretions, When fecal material is unintentionally ingested, infection can occur. Feline zooinosis can be acquired by a person by inhalation of aerosols or droplets coughed up by the cat.

In the United States, forty percent of homes have at least one cat. Some contagious infections such as campylobacteriosis and salmonellosis cause visible symptoms of the disease in cats. Other infections, such as cat scratch disease and toxoplasmosis, have no visible symptoms and are carried by apparently healthy cats.

Neurosyphilis has four different forms: asymptomatic, meningovascular, tabes dorsalis, and general paresis. In rare instances, active neurosyphilis can mimic Alzheimer's disease.

Symptoms of neurosyphilis include, but are not limited to the following:

- Abnormal gait

- Blindness

- Confusion, disorientation

- Sudden personality changes

- Changes in mental stability

- Dementia

- Depression

- Headache

- Fecal and urinary incontinence

- Irritability

- Memory problems

- Mood disturbances

- Numbness in the toes, feet, or legs

- Poor concentration

- Psychosis

- Seizures

- Neck stiffness

- Tremors

- Visual disturbances. There may be the sign of Argyll Robertson pupils, which are bilateral small pupils that constrict when the patient focuses on a near object but do not constrict when exposed to bright light.

- Muscle weakness

Upon further diagnostic workup, the following signs may be present:

- Abnormal reflexes

- Muscle atrophy

- Muscle contractions

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).

An individual may only develop signs of an infection after a period of subclinical infection, a duration that is called the incubation period. This is the case, for example, for subclinical sexually transmitted diseases such as AIDS and genital warts. Individuals with such subclinical infections, and those that never develop overt illness, creates a reserve of individuals that can transmit an infectious agent to infect other individuals. Because such cases of infections do not come to clinical attention, health statistics can often fail to measure the true prevalence of an infection in a population, and this prevents the accurate modeling of its infectious transmission.

Similarities between the conditions chancre and chancroid:

- Both originate as pustules at the site of inoculation, and progress to ulcerated lesions

- Both lesions are typically 1–2 cm in diameter

- Both lesions are caused by sexually transmissible organisms

- Both lesions typically appear on the genitals of infected individuals

In addition to evaluation of any symptoms and signs, various blood tests can be done:

- Venereal Disease Research Laboratory test (VDRL)

- Fluorescent treponemal antibody absorption (FTA-ABS)

- Rapid plasma reagin (RPR)

- Treponema pallidum particle agglutination assay (TPPA)

Also, it is important to test the cerebrospinal fluid for signs of syphilis.

Additional tests to look for problems with the nervous system may include the following:

- Cerebral angiogram

- Head CT scan

- Lumbar puncture ("spinal tap") to acquire a sample for cerebrospinal fluid analysis

- MRI scan of the brain, brainstem, or spinal cord

A vertically transmitted infection is an infection caused by pathogens (such as bacteria and viruses) that uses mother-to-child transmission, that is, transmission directly from the mother to an embryo, fetus, or baby during pregnancy or childbirth. It can occur when the mother gets an infection as an intercurrent disease in pregnancy. Nutritional deficiencies may exacerbate the risks of perinatal infection.

It resembles bacterial sepsis and can occur after initiation of antibacterials, such as mild silver protein, penicillin or tetracycline, for the treatment of louse-borne relapsing fever (80–90% of patients) and in tick-borne relapsing fever (30–40%). It usually manifests within a few hours of the first dose of antibiotic as fever, chills, rigor, hypotension, headache, tachycardia, hyperventilation, vasodilation with flushing, myalgia (muscle pain), exacerbation of skin lesions and anxiety. The intensity of the reaction indicates the severity of inflammation. Reaction commonly occurs within two hours of drug administration, but is usually self-limiting. It is observed in 50% of patients with primary syphilis and about 90% of patients with secondary syphilis.

Aujeszky's disease, usually called pseudorabies in the United States, is a viral disease in swine that has been endemic in most parts of the world. It is caused by "Suid herpesvirus 1" (SuHV1). Aujeszky's disease is considered to be the most economically important viral disease of swine in areas where hog cholera has been eradicated. Other mammals, such as humans, cattle, sheep, goats, cats, dogs, and raccoons, are also susceptible. The disease is usually fatal in these animal species bar humans.

The term "pseudorabies" is found inappropriate by many people, as SuHV1 is a herpesvirus and not related to the rabies virus.

Research on SuHV1 in pigs has pioneered animal disease control with genetically modified vaccines. SuHV1 is now used in model studies of basic processes during lytic herpesvirus infection, and for unravelling molecular mechanisms of herpesvirus neurotropism.