Common complications include pneumonia, bronchitis, encephalopathy, earache, and seizures. Most healthy older children and adults fully recover, but those with comorbid conditions have a higher risk of morbidity and mortality.

Infection in newborns is particularly severe. Pertussis is fatal in an estimated 1.6% of hospitalized US infants under one year of age. First-year infants are also more likely to develop complications, such as: pneumonia (20%), encephalopathy (0.3%), seizures (1%), failure to thrive, and death (1%)—perhaps due to the ability of the bacterium to suppress the immune system. Pertussis can cause severe paroxysm-induced cerebral hypoxia, and 50% of infants admitted to hospital suffer apneas. Reported fatalities from pertussis in infants increased substantially from 1990 to 2010.

Human-to-human transmission of diphtheria typically occurs through the air when an infected individual coughs or sneezes. Breathing in particles released from the infected individual leads to infection Contact with any lesions on the skin can also lead to transmission of diphtheria, but this is uncommon. Indirect infections can occur, as well. If an infected individual touches a surface or object, the bacteria can be left behind and remain viable. Also, some evidence indicates diphtheria has the potential to be zoonotic, but this has yet to be confirmed. "Corynebacterium ulcerans" has been found in some animals, which would suggest zoonotic potential

Diphtheria is fatal in between 5% and 10% of cases. In children under five years and adults over 40 years, the fatality rate may be as much as 20%. In 2013, it resulted in 3,300 deaths, down from 8,000 deaths in 1990.

The number of cases has changed over the course of the last 2 decades, specifically throughout developing countries. Better standards of living, mass immunization, improved diagnosis, prompt treatment, and more effective health care have led to the decrease in cases worldwide. However, although outbreaks are rare, they still occur worldwide, especially in developed nations such as Germany among unvaccinated children, and Canada. After the breakup of the former Soviet Union in the early 1990s, vaccination rates in its constituent countries fell so low that an explosion of diphtheria cases occurred. In 1991, 2,000 cases of diphtheria occurred in the USSR. Because of this outbreak, since 1992, many of the cases reported throughout other parts of Europe have been linked to the NIS epidemic. Belgium (3/3) and Finland (10/10) come in first, stating that 100% of cases are connected to this epidemic. However, locations such as Poland and Germany have had a larger number of people diagnosed with Diphtheria overall, but claim that a smaller percentage have been linked directly to the NIS. By 1998 as many as 200,000 cases in the Commonwealth of Independent States were reported, with 5,000 deaths.

The primary method of prevention for pertussis is vaccination. Evidence is insufficient to determine the effectiveness of antibiotics in those who have been exposed, but are without symptoms. Preventive antibiotics, however, are still frequently used in those who have been exposed and are at high risk of severe disease (such as infants).

The live attenuated BCG vaccine developed against tuberculosis has been shown to have strong beneficial effects on the ability to combat non-tuberculosis infections.

Several studies have suggested that BCG vaccination may reduce atopy, particularly when given early in life. Furthermore, in multiple observational studies BCG vaccination has been shown to provide beneficial effects on overall mortality. These observations encouraged randomised controlled trials to examine BCG vaccination's beneficial non-specific effects on overall health. Since BCG vaccination is recommended to be given at birth in countries that have a high incidence of tuberculosis it would have been unethical to randomize children into 'BCG' vs. 'no BCG' groups. However, many low-income countries delay BCG vaccination for low-birth-weight (LBW) infants; this offered the opportunity to directly test the effect of BCG on overall mortality.

In the first two randomised controlled trials receipt of BCG+OPV at birth vs. OPV only ('delayed BCG') was associated with strong reductions in neonatal mortality; these effects were seen as early as 3 days after vaccination. BCG protected against sepsis as well as respiratory infections.

Among BCG vaccinated children, those who develop a BCG scar or a positive skin test (TST) are less likely to develop sepsis and exhibit an overall reduction in child mortality of around 50%.

In a recent WHO-commissioned review based on five clinical trials and nine observational studies, it was concluded that "the results indicated a beneficial effect of BCG on overall mortality in the first 6–12 months of life. Relevant follow-up in some of the trials was short, and all of the observational studies were regarded as being at risk of bias, so the confidence in the findings was rated as very low according to the GRADE criteria and "There was a suggestion that BCG vaccination may be more beneficial the earlier it is given". Furthermore, "estimated effects are in the region of a halving of mortality risk" and "any effect of BCG vaccine on all-cause mortality is not likely to be attributable to any great extent to fewer deaths from tuberculosis (i.e. to a specific effect of BCG vaccine against tuberculosis)". Based on the evidence, the WHO's Strategic Group of Experts on Immunization concluded that "the non-specific effects on all-cause mortality warrant further research".

Most strains of "H. influenzae" are opportunistic pathogens; that is, they usually live in their host without causing disease, but cause problems only when other factors (such as a viral infection, reduced immune function or chronically inflamed tissues, e.g. from allergies) create an opportunity. They infect the host by sticking to the host cell using trimeric autotransporter adhesins.

Naturally acquired disease caused by "H. influenzae" seems to occur in humans only. In infants and young children, "H. influenzae" type b (Hib) causes bacteremia, pneumonia, epiglottitis and acute bacterial meningitis. On occasion, it causes cellulitis, osteomyelitis, and infectious arthritis. It is one cause of neonatal infection.

Due to routine use of the Hib conjugate vaccine in the U.S. since 1990, the incidence of invasive Hib disease has decreased to 1.3/100,000 in children. However, Hib remains a major cause of lower respiratory tract infections in infants and children in developing countries where the vaccine is not widely used. Unencapsulated "H. influenzae" strains are unaffected by the Hib vaccine and cause ear infections (otitis media), eye infections (conjunctivitis), and sinusitis in children, and are associated with pneumonia.

Standard titer measles vaccine is recommended at 9 months of age in low-income countries where measles infection is endemic and often fatal. Many observational studies have shown that measles-vaccinated children have substantially lower mortality than can be explained by the prevention of measles-related deaths. Many of these observational studies were natural experiments, such as studies comparing the mortality before and after the introduction of measles vaccine and other studies where logistical factors rather than maternal choice determined whether a child was vaccinated or not.

These findings were later supported in randomized trials from 2003 to 2009 in Guinea-Bissau. An intervention group of children given standard titer measles vaccine at 4.5 and 9 month of age had a 30% reduction in all-cause mortality compared to the children in the control group, which were only vaccinated against measles at 9 month of age.

In a recent WHO-commissioned review based on four randomized trials and 18 observational studies, it was concluded that "There was consistent evidence of a beneficial effect of measles vaccine, although all observational studies were assessed as being at risk of bias and the GRADE rating was of low confidence. There was an apparent difference between the effect in girls and boys, with girls benefitting more from measles vaccination", and furthermore "estimated effects are in the region of a halving of mortality risk" and "if these effects are real then they are not fully explained by deaths that were established as due to measles". Based on the evidence, the WHO's Strategic Advisory Group of Experts on Immunization concluded that "the non-specific effects on all-cause mortality warrant further research".

Many cases of croup have been prevented by immunization for influenza and diphtheria. At one time, croup referred to a diphtherial disease, but with vaccination, diphtheria is now rare in the developed world.

Croup affects about 15% of children, and usually presents between the ages of 6 months and 5–6 years. It accounts for about 5% of hospital admissions in this population. In rare cases, it may occur in children as young as 3 months and as old as 15 years. Males are affected 50% more frequently than are females, and there is an increased prevalence in autumn.

In 1930, two major categories of "H. influenzae" were defined: the unencapsulated strains and the encapsulated strains. Encapsulated strains were classified on the basis of their distinct capsular antigens. There are six generally recognized types of encapsulated "H. influenzae": a, b, c, d, e, and f.

Genetic diversity among unencapsulated strains is greater than within the encapsulated group. Unencapsulated strains are termed nontypable (NTHi) because they lack capsular serotypes; however, they can be classified by multilocus sequence typing. The pathogenesis of "H. influenzae" infections is not completely understood, although the presence of the capsule in encapsulated type b (Hib), a serotype causing conditions such as epiglottitis, is known to be a major factor in virulence. Their capsule allows them to resist phagocytosis and complement-mediated lysis in the nonimmune host. The unencapsulated strains are almost always less invasive; they can, however, produce an inflammatory response in humans, which can lead to many symptoms. Vaccination with Hib conjugate vaccine is effective in preventing Hib infection, but does not prevent infection with NTHi strains.

The impact of the pandemic and its mortality rate are different for men and women. Mortality is higher in men in studies conducted in China and Italy. The highest risk for men is in their 50s, with the gap between men and women closing only at 90. In China, the death rate was 2.8 percent for men and 1.7 percent for women. The exact reasons for this sex-difference is not known, but genetic and behavioural factors could be a reason. Sex-based immunological differences, lesser prevalence of smoking in women and men developing co-morbid conditions such as hypertension at a younger age than women could have contributed to the higher mortality in men. In Europe, 57% of the infected individuals were men and 72% of those died with COVID-19 were men. As of April 2020, the US government is not tracking sex-related data of COVID-19 infections. Research has shown that viral illnesses like Ebola, HIV, influenza and SARS affect men and women differently. A higher percentage of health workers, particularly nurses, are women, and they have a higher chance of being exposed to the virus. School closures, lockdowns and reduced access to healthcare following the 2019–20 coronavirus pandemic may deferentially affect the genders and possibly exaggerate existing gender disparity.

As of March 2020, it was unknown if past infection provides effective and long-term immunity in people who recover from the disease. Immunity is seen as likely, based on the behaviour of other coronaviruses, but cases in which recovery from COVID-19 have been followed by positive tests for coronavirus at a later date have been reported. These cases are believed to be worsening of a lingering infection rather than re-infection.

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).

Tetanus is caused by the tetanus bacterium "Clostridium tetani". Tetanus is an international health problem, as "C. tetani" spores are ubiquitous. Spores can be introduced into the body through a puncture wound (penetrating trauma). Due to "C. tetani" being an anaerobic bacterium, it and its endospores thrive in environments that lack oxygen, such as a puncture wound.

The disease occurs almost exclusively in persons inadequately immunized. It is more common in hot, damp climates with soil rich in organic matter. Manure-treated soils may contain spores, as they are widely distributed in the intestines and feces of many animals such as horses, sheep, cattle, dogs, cats, rats, guinea pigs, and chickens. In agricultural areas, a significant number of human adults may harbor the organism.

The spores can also be found on skin surfaces and in contaminated heroin. Heroin users, particularly those that inject the drug subcutaneously, appear to be at high risk of contracting tetanus. Rarely, tetanus can be contracted through surgical procedures, intramuscular injections, compound fractures, and dental infections. The bite of a dog can transmit tetanus.

Tetanus is often associated with rust, especially rusty nails. Although rust itself does not cause tetanus, objects that accumulate rust are often found outdoors or in places that harbour anaerobic bacteria. Additionally, the rough surface of rusty metal provides a habitat for "C. tetani", while a nail affords a means to puncture skin and deliver endospores deep within the body at the site of the wound. An endospore is a non-metabolizing survival structure that begins to metabolize and cause infection once in an adequate environment. Hence, stepping on a nail (rusty or not) may result in a tetanus infection, as the low-oxygen (anaerobic) environment may exist under the skin, and the puncturing object can deliver endospores to a suitable environment for growth.

Vaccination helps prevent bronchopneumonia, mostly against influenza viruses, adenoviruses, measles, rubella, streptococcus pneumoniae, haemophilus influenzae, diphtheria, bacillus anthracis, chickenpox, and bordetella pertussis.

Unlike many infectious diseases, recovery from naturally acquired tetanus does not usually result in immunity to tetanus. This is due to the extreme potency of the tetanospasmin toxin. Tetanospasmin will likely be lethal before it will provoke an immune response.

Tetanus can be prevented by vaccination with tetanus toxoid. The CDC recommends that adults receive a booster vaccine every ten years, and standard care practice in many places is to give the booster to any patient with a puncture wound who is uncertain of when he or she was last vaccinated, or if he or she has had fewer than three lifetime doses of the vaccine. The booster may not prevent a potentially fatal case of tetanus from the current wound, however, as it can take up to two weeks for tetanus antibodies to form.

In children under the age of seven, the tetanus vaccine is often administered as a combined vaccine, DPT/DTaP vaccine, which also includes vaccines against diphtheria and pertussis. For adults and children over seven, the Td vaccine (tetanus and diphtheria) or Tdap (tetanus, diphtheria, and acellular pertussis) is commonly used.

The World Health Organization certifies countries as having eliminated maternal or neonatal tetanus. Certification requires at least two years of rates of less than 1 case per 1000 live births. In 1998 in Uganda, 3,433 tetanus cases were recorded in newborn babies; of these, 2,403 died. After a major public health effort, Uganda in 2011 was certified as having eliminated tetanus.

Since human plague is rare in most parts of the world, routine vaccination is not needed other than for those at particularly high risk of exposure, nor for people living in areas with enzootic plague, meaning it occurs at regular, predictable rates in populations and specific areas, such as the western United States. It is not even indicated for most travellers to countries with known recent reported cases, particularly if their travel is limited to urban areas with modern hotels. The CDC thus only recommends vaccination for: (1) all laboratory and field personnel who are working with "Y. pestis" organisms resistant to antimicrobials; (2) people engaged in aerosol experiments with "Y. pestis"; and (3) people engaged in field operations in areas with enzootic plague where preventing exposure is not possible (such as some disaster areas).

A systematic review by the Cochrane Collaboration found no studies of sufficient quality to make any statement on the efficacy of the vaccine.

Due to the importance of disease caused by "S. pneumoniae" several vaccines have been developed to protect against invasive infection. The World Health Organization recommend routine childhood pneumococcal vaccination; it is incorporated into the childhood immunization schedule in a number of countries including the United Kingdom, United States, and South Africa.

There has been evidence of limited, but not sustained spread of MERS-CoV from person to person, both in households as well as in health care settings like hospitals. Most transmission has occurred "in the circumstances of close contact with severely ill persons in healthcare or household settings" and there is no evidence of transmission from asymptomatic cases. Cluster sizes have ranged from 1 to 26 people, with an average of 2.7.

The WHO lists 25 diseases for which vaccines are available:

1. Measles

2. Rubella

3. Cholera

4. Meningococcal disease

5. Influenza

6. Diphtheria

7. Mumps

8. Tetanus

9. Hepatitis A

10. Pertussis

11. Tuberculosis

12. Hepatitis B

13. Pneumoccocal disease

14. Typhoid fever

15. Hepatitis E

16. Poliomyelitis

17. Tick-borne encephalitis

18. Haemophilus influenzae type b

19. Rabies

20. Varicella and herpes zoster (shingles)

21. Human papilloma-virus

22. Rotavirus gastroenteritis

23. Yellow fever

24. Japanese encephalitis

25. Malaria

26. Dengue fever

"S. pneumoniae" is responsible for 15–50% of all episodes of community acquired pneumonia, 30–50% of all cases of acute otitis media, and a significant proportion of bloodstream infections and bacterial meningitis.

As estimated by WHO in 2005 it killed about 1.6 million children every year worldwide with 0.7–1 million of them being under the age of five. The majority of these deaths were in developing countries.

Lower respiratory infectious disease is the fifth-leading cause of death and the combined leading infectious cause of death, being responsible for 2·74 million deaths worldwide. This is generally similar to estimates in the 2010 Global Burden of Disease study.

This total only accounts for "Streptococcus pneumoniae" and "Haemophilus Influenzae" infections and does not account for atypical or nosocomial causes of lower respiratory disease, therefore underestimating total disease burden.

Transmission of "Y. pestis" to an uninfected individual is possible by any of the following means.

- droplet contact – coughing or sneezing on another person

- direct physical contact – touching an infected person, including sexual contact

- indirect contact – usually by touching soil contamination or a contaminated surface

- airborne transmission – if the microorganism can remain in the air for long periods

- fecal-oral transmission – usually from contaminated food or water sources

- vector borne transmission – carried by insects or other animals.

"Yersinia pestis" circulates in animal reservoirs, particularly in rodents, in the natural foci of infection found on all continents except Australia. The natural foci of plague are situated in a broad belt in the tropical and sub-tropical latitudes and the warmer parts of the temperate latitudes around the globe, between the parallels 55 degrees North and 40 degrees South.

Contrary to popular belief, rats did not directly start the spread of the bubonic plague. It is mainly a disease in the fleas ("Xenopsylla cheopis") that infested the rats, making the rats themselves the first victims of the plague. Infection in a human occurs when a person is bitten by a flea that has been infected by biting a rodent that itself has been infected by the bite of a flea carrying the disease. The bacteria multiply inside the flea, sticking together to form a plug that blocks its stomach and causes it to starve. The flea then bites a host and continues to feed, even though it cannot quell its hunger, and consequently the flea vomits blood tainted with the bacteria back into the bite wound. The bubonic plague bacterium then infects a new person and the flea eventually dies from starvation. Serious outbreaks of plague are usually started by other disease outbreaks in rodents, or a rise in the rodent population.

Hand, foot and mouth disease most commonly occurs in children under the age of 10. It tends to occur in outbreaks during the spring, summer, and autumn seasons. This is believed to be due to heat and humidity improving spread. HFMD is more common in rural areas than urban areas, however, socioeconomic status and hygiene levels need to considered. Poor hygiene is a risk factor for HFMD.

Outbreaks have relatively recently in China, Japan, Hong Kong, the Republic of Korea, Malaysia, Singapore, Thailand, Taiwan and Vietnam. HFMD most commonly affects young children under the age of 10 and more often under the age of 5, but can also affect adults with varying symptoms.

Since 1997 there have been 71 large enterovirus outbreaks reported, mostly in East and South East Asia, primarily affecting children. From the years 2008 to 2014, more than 1 million HFMD cases have been reported in China each year.

A drug-resistant strain of scarlet fever, resistant to macrolide antibiotics such as erythromycin, but retaining drug-sensitivity to beta-lactam antibiotics such as penicillin, emerged in Hong Kong in 2011, accounting for at least two deaths in that city—the first such in over a decade. About 60% of circulating strains of the group A "Streptococcus" which cause scarlet fever in Hong Kong are resistant to macrolide antibiotics, says Professor Kwok-yung Yuen, head of Hong Kong University's microbiology department. Previously, observed resistance rates had been 10–30%; the increase is likely the result of overuse of macrolide antibiotics in recent years.