Persons with component deficiencies in the final common complement pathway (C3,C5-C9) are more susceptible to "N. meningitidis" infection than complement-satisfactory persons, and it was estimated that the risk of infection is 7000 times higher in such individuals. In addition, complement component-deficient populations frequently experience frequent meningococcal disease since their immune response to natural infection may be less complete than that of complement non-deficient persons.

Inherited properdin deficiency also is related, with an increased risk of contracting meningococcal disease. Persons with functional or anatomic asplenia may not efficiently clear encapsulated "Neisseria meningitidis" from the bloodstream Persons with other conditions associated with immunosuppression also may be at increased risk of developing meningococcal disease.

About 33 million people are affected by rheumatic heart disease with an additional 47 million having asymptomatic damage to their heart valves. As of 2010 globally it resulted in 345,000 deaths, down from 463,000 in 1990.

In Western countries, rheumatic fever has become fairly rare since the 1960s, probably due to the widespread use of antibiotics to treat streptococcus infections. While it has been far less common in the United States since the beginning of the 20th century, there have been a few outbreaks since the 1980s. Although the disease seldom occurs, it is serious and has a case-fatality rate of 2–5%.

Rheumatic fever primarily affects children between ages 5 and 17 years and occurs approximately 20 days after strep throat. In up to a third of cases, the underlying strep infection may not have caused any symptoms.

The rate of development of rheumatic fever in individuals with untreated strep infection is estimated to be 3%. The incidence of recurrence with a subsequent untreated infection is substantially greater (about 50%). The rate of development is far lower in individuals who have received antibiotic treatment. Persons who have suffered a case of rheumatic fever have a tendency to develop flare-ups with repeated strep infections.

The recurrence of rheumatic fever is relatively common in the absence of maintenance of low dose antibiotics, especially during the first three to five years after the first episode. Recurrent bouts of rheumatic fever can lead to valvular heart disease. Heart complications may be long-term and severe, particularly if valves are involved. In countries in Southeast-Asia, sub-saharan Africa, and Oceania, the percentage of people with rheumatic heart disease detected by listening to the heart was 2.9 per 1000 children and by echocardiography it was 12.9 per 1000 children.

It is recommended that primary immunization against meningococcal disease with meningitis A,C,Y and W-135 vaccines for all young adolescents at 11–12 years of age and all unvaccinated older adolescents at 15 years of age. Although conjugate vaccines are the preferred meningococcal vaccine in adolescents 11 years of age or older, polysaccharide vaccines are an acceptable alternative if the conjugated vaccine is unavailable.

Some strains of group A streptococci (GAS) cause severe infection. Severe infections are usually invasive, meaning that the bacteria has entered parts of the body where bacteria are not usually found, such as the blood, lungs, deep muscle or fat tissue. Those at greatest risk include children with chickenpox; persons with suppressed immune systems; burn victims; elderly persons with cellulitis, diabetes, vascular disease, or cancer; and persons taking steroid treatments or chemotherapy. Intravenous drug users also are at high risk. GAS is an important cause of puerperal fever worldwide, causing serious infection and, if not promptly diagnosed and treated, death in newly delivered mothers. Severe GAS disease may also occur in healthy persons with no known risk factors.

All severe GAS infections may lead to shock, multisystem organ failure, and death. Early recognition and treatment are critical. Diagnostic tests include blood counts and urinalysis as well as cultures of blood or fluid from a wound site.

Severe Group A streptococcal infections often occur sporadically but can be spread by person-to-person contact.

Public Health policies internationally reflect differing views of how the close contacts of people affected by severe Group A streptococcal infections should be treated. Health Canada and the US CDC recommend close contacts see their doctor for full evaluation and may require antibiotics; current UK Health Protection Agency guidance is that, for a number of reasons, close contacts should not receive antibiotics unless they are symptomatic but that they should receive information and advice to seek immediate medical attention if they develop symptoms. However, guidance is clearer in the case of mother-baby pairs: both mother and baby should be treated if either develops an invasive GAS infection within the first 28 days following birth (though some evidence suggests that this guidance is not routinely followed in the UK).

Untreated, bacterial meningitis is almost always fatal. Viral meningitis, in contrast, tends to resolve spontaneously and is rarely fatal. With treatment, mortality (risk of death) from bacterial meningitis depends on the age of the person and the underlying cause. Of newborns, 20–30% may die from an episode of bacterial meningitis. This risk is much lower in older children, whose mortality is about 2%, but rises again to about 19–37% in adults. Risk of death is predicted by various factors apart from age, such as the pathogen and the time it takes for the pathogen to be cleared from the cerebrospinal fluid, the severity of the generalized illness, a decreased level of consciousness or an abnormally low count of white blood cells in the CSF. Meningitis caused by "H. influenzae" and meningococci has a better prognosis than cases caused by group B streptococci, coliforms and "S. pneumonia". In adults, too, meningococcal meningitis has a lower mortality (3–7%) than pneumococcal disease.

In children there are several potential disabilities which may result from damage to the nervous system, including sensorineural hearing loss, epilepsy, learning and behavioral difficulties, as well as decreased intelligence. These occur in about 15% of survivors. Some of the hearing loss may be reversible. In adults, 66% of all cases emerge without disability. The main problems are deafness (in 14%) and cognitive impairment (in 10%).

Tuberculous meningitis in children continues to be associated with a significant risk of death even with treatment (19%), and a significant proportion of the surviving children have ongoing neurological problems. Just over a third of all cases survives with no problems.

Some patients develop significant carditis which manifests as congestive heart failure. This requires the usual treatment for heart failure: ACE inhibitors, diuretics, beta blockers, and digoxin. Unlike normal heart failure, rheumatic heart failure responds well to corticosteroids.

Group A β-hemolytic streptococcus can cause infections of the throat and skin. These may vary from very mild conditions to severe, life-threatening diseases. Although it is not completely clear what causes different people to develop different diseases as a result of infection with the same pathogenic bacteria, it is suspected that host phenotypic and epigenetic factors are the source of such variation. Indeed, the many virulence factors of GAS can influence the epigenetics of the host. Furthermore, persons with suppressed or compromised immune systems may be more susceptible to certain diseases caused by GAS than other persons with intact immune systems.

Humans may also carry the GAS either on the skin or in the throat and show no symptoms. These carriers are less contagious than symptomatic carriers of the bacteria.

The non-invasive infections caused by GAS tend to be less severe and more common. They occur when the bacteria colonizes the throat area, where it recognizes epithelial cells. The two most prominent infections of GAS are both non-invasive: strep throat (pharyngitis) where it causes 15- 30% of the childhood cases and 10% of adult cases, and impetigo. These may be effectively treated with antibiotics. Scarlet fever is also a non-invasive infection caused by GAS, although much less common.

The invasive infections caused by Group A β-hemolytic streptococcus tend to be more severe and less common. These occurs when the bacterium is able to infect areas where bacteria are not usually found, such as blood and organs. The diseases that may be caused as a result of this include streptococcal toxic shock syndrome (STSS), necrotizing fasciitis (NF), pneumonia, and bacteremia.

In addition, infection of GAS may lead to further complications and health conditions, namely acute rheumatic fever and poststreptococcal glomerulonephritis.

Most Common:

- impetigo, cellulitis, and erysipelas – infections of the skin which can be complicated by necrotizing fasciitis – skin, fascia and muscle

- strep throat AKA strep pharyngitis – pharynx

Less Common:

- Bacteremia can be associated with these infections, but is not typical.

- septic arthritis – joints

- osteomyelitis – bones

- vaginitis – vagina (more common in pre-pubescent girls)

- meningitis* – meninges

- sinusitis* – sinuses

- pneumonia* – pulmonary alveolus

Bacterial and viral meningitis are contagious, but neither is as contagious as the common cold or flu. Both can be transmitted through droplets of respiratory secretions during close contact such as kissing, sneezing or coughing on someone, but cannot be spread by only breathing the air where a person with meningitis has been. Viral meningitis is typically caused by enteroviruses, and is most commonly spread through fecal contamination. The risk of infection can be decreased by changing the behavior that led to transmission.

Pericarditis may be caused by viral, bacterial, or fungal infection.

In the developed world viruses are believed to be the cause of about 85% of cases. In the developing world tuberculosis is a common cause but it is rare in the developed world.

Viral causes include coxsackievirus, herpesvirus, mumps virus, and HIV among others.

Pneumococcus or tuberculous pericarditis are the most common bacterial forms. Anaerobic bacteria can also be a rare cause. Fungal pericarditis is usually due to histoplasmosis, or in immunocompromised hosts Aspergillus, Candida, and Coccidioides. The most common cause of pericarditis worldwide is infectious pericarditis with tuberculosis.

When properly diagnosed, the mortality of Lemierre's syndrome is about 4.6%. Since this disease is not well known and often remains undiagnosed, mortality might be much higher.

About 30% of people with viral pericarditis or pericarditis of an unknown cause have one or several recurrent episodes.

The bacteria causing the thrombophlebitis are anaerobic bacteria that are typically normal components of the microorganisms that inhabit the mouth and throat. Species of "Fusobacterium", specifically "Fusobacterium necrophorum", are most commonly the causative bacteria, but various bacteria have been implicated. One 1989 study found that 81% of Lemierres's syndrome had been infected with "Fusobacterium necrophorum", while 11% were caused by other Fusobacterium species. MRSA might also be an issue in Lemierre infections. Rarely Lemierre's syndrome is caused by other (usually Gram-negative) bacteria, which include "Bacteroides fragilis" and "Bacteroides melaninogenicus", "Peptostreptococcus spp.", "Streptococcus microaerophile", "Staphylococcus aureus", "Streptococcus pyogenes", and "Eikenella corrodens".

There are several causes of acute pericarditis. In developed nations, the cause of most (80–90%) cases of acute pericarditis is unknown but a viral cause is suspected in the majority of such cases. The other 10–20% of acute pericarditis cases have various causes including connective tissue diseases (e.g., systemic lupus erythematosus), cancer, or involve an inflammatory reaction of the pericardium following trauma to the heart such as after a heart attack such as Dressler's syndrome. Familial mediterranean fever and TNF receptor associated periodic syndrome are rare inherited autoimmune diseases capable of causing recurring episodes of acute pericarditis.

It is estimated that seven to ten million people are infected by leptospirosis annually. One million cases of severe leptospirosis occur annually, with 58,900 deaths. Annual rates of infection vary from 0.02 per 100,000 in temperate climates to 10 to 100 per 100,000 in tropical climates. This leads to a lower number of registered cases than likely exists.

The number of new cases of leptospirosis is difficult to estimate since many cases of the disease go unreported. There are many reasons for this, but the biggest issue is separating the disease from other similar conditions. Laboratory testing is lacking in many areas.

In context of global epidemiology, the socioeconomic status of many of the world’s population is closely tied to malnutrition; subsequent lack of micronutrients may lead to increased risk of infection and death due to leptospirosis infection. Micronutrients such as iron, calcium, and magnesium represent important areas of future research.

Outbreaks that occurred after the 1940's have happened mostly in the late summer seasons, which happens to be the driest part of the year. The people at the highest risk for leptospirosis are young people whose age ranges from 5-16 years old, and can also range to young adults.

The amount of cases increase during the rainy season in the tropics and during the late summer or early fall in Western countries. This happens because leptospires survive best in fresh water, damp alkaline soil, vegetation, and mud with temperatures higher that 22° C. This also leads to increased risk of exposure to populations during flood conditions, and leptospire concentrations to peak in isolated pools during drought. There is no evidence of leptospirosis having any effect on sexual and age-related differences. However, a major risk factor for development of the disease is occupational exposure, a disproportionate number of working-aged males are affected. There have been reported outbreaks where more than 40% of people are younger than 15. “Active surveillance measures have detected leptospire antibodies in as many as 30% of children in some urban American populations.” Potential reasons for such cases include children playing with suspected vectors such as dogs or indiscriminate contact with water.

Doxycycline has been provided once a week as a prophylaxis to minimize infections during outbreaks in endemic regions. However, there is no evidence that chemoprophylaxis is effective in containing outbreaks of leptospirosis, and use of antibiotics increases antibiotics resistance. Pre-exposure prophylaxis may be beneficial for individuals traveling to high-risk areas for a short stay.

Effective rat control and avoidance of urine contaminated water sources are essential preventive measures. Human vaccines are available only in a few countries, such as Cuba and China. Animal vaccines only cover a few strains of the bacteria. Dog vaccines are effective for at least one year.

Serious complications are uncommon, occurring in less than 5% of cases:

- CNS complications include meningitis, encephalitis, hemiplegia, Guillain–Barré syndrome, and transverse myelitis. Prior infectious mononucleiosis has been linked to the development of multiple sclerosis (MS).

- Hematologic: Hemolytic anemia (direct Coombs test is positive) and various cytopenias, and bleeding (caused by thrombocytopenia) can occur.

- Mild jaundice

- Hepatitis with the Epstein–Barr virus is rare.

- Upper airway obstruction from tonsillar hypertrophy is rare.

- Fulminant disease course of immunocompromised patients is rare.

- Splenic rupture is rare.

- Myocarditis and pericarditis are rare.

- Postural orthostatic tachycardia syndrome

- Chronic fatigue syndrome

- Cancers associated with the Epstein-Barr virus include: Burkitt's lymphoma, Hodgkin's lymphoma and lymphomas in general as well as nasopharyngeal and gastric carcinoma.

Once the acute symptoms of an initial infection disappear, they often do not return. But once infected, the patient carries the virus for the rest of his or her life. The virus typically lives dormantly in B lymphocytes. Independent infections of mononucleosis may be contracted multiple times, regardless of whether the patient is already carrying the virus dormantly. Periodically, the virus can reactivate, during which time the patient is again infectious, but usually without any symptoms of illness. Usually, a patient has few, if any, further symptoms or problems from the latent B lymphocyte infection. However, in susceptible hosts under the appropriate environmental stressors, the virus can reactivate and cause vague physical symptoms (or may be subclinical), and during this phase the virus can spread to others.

About 90% of cases of infectious mononucleosis are caused by the Epstein–Barr virus, a member of the Herpesviridae family of DNA viruses. It is one of the most commonly found viruses throughout the world. Contrary to common belief, the Epstein–Barr virus is not highly contagious. It can only be contracted through direct contact with an infected person’s saliva, such as through kissing or sharing toothbrushes, cups, etc. About 95% of the population has been exposed to this virus by the age of 40, but only 15–20% of teenagers and about 40% of exposed adults actually become infected.

Clinical presentation of diseases of pericardium may vary between:

- Acute and recurrent pericarditis

- Pericardial effusion without major hemodynamic compromise

- Cardiac tamponade

- Constrictive pericarditis

- Effusive-constrictive pericarditis

Multiple species of bacteria can be associated with the condition:

- Meningococcus is another term for the bacterial species "Neisseria meningitidis"; blood infection with said species usually underlies WFS. While many infectious agents can infect the adrenals, an acute, selective infection is usually meningococcus.

- "Pseudomonas aeruginosa" can also cause WFS.

- WFS can also be caused by "Streptococcus pneumoniae" infections, a common bacterial pathogen typically associated with meningitis in the adult and elderly population.

- "Mycobacterium tuberculosis" could also cause WFS. Tubercular invasion of the adrenal glands could cause hemorrhagic destruction of the glands and cause mineralocorticoid deficiency.

- "Staphylococcus aureus" has recently also been implicated in pediatric WFS.

- It can also be associated with "Haemophilus influenzae".

Viruses may also be implicated in adrenal problems:

- Cytomegalovirus can cause adrenal insufficiency, especially in the immunocompromised.

- Ebola virus infection may also cause similar acute adrenal failure.

In 2012, the World Health Organization estimated that vaccination prevents 2.5 million deaths each year. If there is 100% immunization, and 100% efficacy of the vaccines, one out of seven deaths among young children could be prevented, mostly in developing countries, making this an important global health issue. Four diseases were responsible for 98% of vaccine-preventable deaths: measles, "Haemophilus influenzae" serotype b, pertussis, and neonatal tetanus.

The Immunization Surveillance, Assessment and Monitoring program of the WHO monitors and assesses the safety and effectiveness of programs and vaccines at reducing illness and deaths from diseases that could be prevented by vaccines.

Vaccine-preventable deaths are usually caused by a failure to obtain the vaccine in a timely manner. This may be due to financial constraints or to lack of access to the vaccine. A vaccine that is generally recommended may be medically inappropriate for a small number of people due to severe allergies or a damaged immune system. In addition, a vaccine against a given disease may not be recommended for general use in a given country, or may be recommended only to certain populations, such as young children or older adults. Every country makes its own vaccination recommendations, based on the diseases that are common in its area and its healthcare priorities. If a vaccine-preventable disease is uncommon in a country, then residents of that country are unlikely to receive a vaccine against it. For example, residents of Canada and the United States do not routinely receive vaccines against yellow fever, which leaves them vulnerable to infection if travelling to areas where risk of yellow fever is highest (endemic or transitional regions).

A large number of causes of myocarditis have been identified, but often a cause cannot be found. In Europe and North America, viruses are common culprits. Worldwide, however, the most common cause is Chagas' disease, an illness endemic to Central and South America that is due to infection by the protozoan "Trypanosoma cruzi". Many of the causes listed below, particularly those involving protozoa, fungi, parasites, allergy, autoimmune disorders, and drugs are also causes of eosinophilic myocarditis.

The cause of constrictive pericarditis in the developing world are idiopathic in origin, though likely infectious in nature. In regions where tuberculosis is common, it is the cause in a large portion of cases.

Causes of constrictive pericarditis include:

- Tuberculosis

- Incomplete drainage of purulent pericarditis

- Fungal and parasitic infections

- Chronic pericarditis

- Postviral pericarditis

- Postsurgical

- Following MI, post-myocardial infarction

- In association with pulmonary asbestos

Bacterial myocarditis is rare in patients without immunodeficiency.

Uremic pericarditis is a form of pericarditis. It causes fibrinous pericarditis. The main cause of the disease is poorly understood.

Fibrinous pericarditis is an exudative inflammation. The pericardium is infiltrated by the fibrinous exudate. This consists of fibrin strands and leukocytes. Fibrin describes an amorphous, eosinophilic (pink) network. Leukocytes (white blood cells; mainly neutrophils) are found within the fibrin deposits and intrapericardic. Vascular congestion is also present. Inflammatory cells do not penetrate the myocardium (as is seen with other presentations of pericarditis), and as a result, this particular variant does not present with diffuse ST elevation on ECG (a classic sign of pericarditis known as stage I ECG changes which are seen with other causes). To naked eye examination, this pathology is referred to as having a "Bread and Butter Appearance".