New or progressive infiltrate on the chest X-ray with one of the following:

- Fever > 37.8 °C (100 °F)

- Purulent sputum

- Leukocytosis > 10,000 cells/μl

In an elderly person, the first sign of hospital-acquired pneumonia may be mental changes or confusion.

Other symptoms may include:

- A cough with greenish or pus-like phlegm (sputum)

- Fever and chills

- General discomfort, uneasiness, or ill feeling (malaise)

- Loss of appetite

- Nausea and vomiting

- Sharp chest pain that gets worse with deep breathing or coughing

- Shortness of breath

- Decreased blood pressure and fast heart rate

"Klebsiella" pneumonia (KP) is a form of bacterial pneumonia associated with "Klebsiella pneumoniae". It is typically due to aspiration and alcoholism may be a risk factor, though it is also commonly implicated in hospital-acquired urinary tract infections, and COPD(chronic obstructive pulmonary disease) individuals

Individuals with "Klebsiella" pneumonia tend to cough up a characteristic sputum, as well as having fever, nausea, tachycardia and vomiting. "Klebsiella" pneumonia tends to affect people with underlying conditions, such as alcoholism.

Bacterial pneumonia is a type of pneumonia caused by bacterial infection.

"Streptococcus pneumoniae" () is the most common bacterial cause of pneumonia in all age groups except newborn infants. "Streptococcus pneumoniae" is a Gram-positive bacterium that often lives in the throat of people who do not have pneumonia.

Other important Gram-positive causes of pneumonia are "Staphylococcus aureus" () and "Bacillus anthracis".

Community-acquired pneumonia (CAP) refers to pneumonia (any of several lung diseases) contracted by a person with little contact with the healthcare system. The chief difference between hospital-acquired pneumonia (HAP) and CAP is that patients with HAP live in long-term care facilities or have recently visited a hospital. CAP is common, affecting people of all ages, and its symptoms occur as a result of oxygen-absorbing areas of the lung (alveoli) filling with fluid. This inhibits lung function, causing dyspnea, fever, chest pains and cough.

CAP, the most common type of pneumonia, is a leading cause of illness and death worldwide. Its causes include bacteria, viruses, fungi and parasites. CAP is diagnosed by assessing symptoms, making a physical examination and on x-ray. Other tests, such as sputum examination, supplement chest x-rays. Patients with CAP sometimes require hospitalization, and it is treated primarily with antibiotics, antipyretics and cough medicine. Some forms of CAP can be prevented by vaccination and by abstaining from tobacco products.

Hospital-acquired pneumonia (HAP) or nosocomial pneumonia refers to any pneumonia contracted by a patient in a hospital at least 48–72 hours after being admitted. It is thus distinguished from community-acquired pneumonia. It is usually caused by a bacterial infection, rather than a virus.

HAP is the second most common nosocomial infection (after urinary tract infections) and accounts for 15–20% of the total. It is the most common cause of death among nosocomial infections and is the primary cause of death in intensive care units.

HAP typically lengthens a hospital stay by 1–2 weeks.

Over 100 microorganisms can cause CAP, with most cases caused by "Streptococcus pneumoniae". Certain groups of people are more susceptible to CAP-causing pathogens; for example, infants, adults with chronic conditions (such as chronic obstructive pulmonary disease), senior citizens, alcoholics and others with compromised immune systems are more likely to develop CAP from "Haemophilus influenzae" or "Pneumocystis carinii". A definitive cause is identified in only half the cases.

A hospital-acquired infection (HAI), also known as a nosocomial infection, is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is sometimes instead called a health care–associated infection (HAI or HCAI). Such an infection can be acquired in hospital, nursing home, rehabilitation facility, outpatient clinic, or other clinical settings. Infection is spread to the susceptible patient in the clinical setting by various means. Health care staff can spread infection, in addition to contaminated equipment, bed linens, or air droplets. The infection can originate from the outside environment, another infected patient, staff that may be infected, or in some cases, the source of the infection cannot be determined. In some cases the microorganism originates from the patient's own skin microbiota, becoming opportunistic after surgery or other procedures that compromise the protective skin barrier. Though the patient may have contracted the infection from their own skin, the infection is still considered nosocomial since it develops in the health care setting.

In the United States, the Centers for Disease Control and Prevention estimated roughly 1.7 million hospital-associated infections, from all types of microorganisms, including bacteria and fungi combined, cause or contribute to 99,000 deaths each year. In Europe, where hospital surveys have been conducted, the category of gram-negative infections are estimated to account for two-thirds of the 25,000 deaths each year. Nosocomial infections can cause severe pneumonia and infections of the urinary tract, bloodstream and other parts of the body. Many types are difficult to treat with antibiotics. In addition, antibiotic resistance can complicate treatment.

Bacteremia is the presence of bacteria in the bloodstream that are alive and capable of reproducing. It is a type of bloodstream infection. Bacteremia is defined as either a primary or secondary process. In primary bacteremia, bacteria have been directly introduced into the bloodstream. Injection drug use may lead to primary bacteremia. In the hospital setting, use of blood vessel catheters contaminated with bacteria may also lead to primary bacteremia. Secondary bacteremia occurs when bacteria have entered the body at another site, such as the cuts in the skin, or the mucous membranes of the lungs (respiratory tract), mouth or intestines (gastrointestinal tract), bladder (urinary tract), or genitals. Bacteria that have infected the body at these sites may then spread into the lymphatic system and gain access to the bloodstream, where further spread can occur.

Bacteremia may also be defined by the timing of bacteria presence in the bloodstream: transient, intermittent, or persistent. In transient bacteremia, bacteria are present in the bloodstream for minutes to a few hours before being cleared from the body, and the result is typically harmless in healthy people. This can occur after manipulation of parts of the body normally colonized by bacteria, such as the mucosal surfaces of the mouth during teeth brushing, flossing, or dental procedures, or instrumentation of the bladder or colon. Intermittent bacteremia is characterized by periodic seeding of the same bacteria into the bloodstream by an existing infection elsewhere in the body, such as an abscess, pneumonia, or bone infection, followed by clearing of that bacteria from the bloodstream. This cycle will often repeat until the existing infection is successfully treated. Persistent bacteremia is characterized by the continuous presence of bacteria in the bloodstream. It is usually the result of an infected heart valve, a central line-associated bloodstream infection (CLABSI), an infected blood clot (suppurative thrombophlebitis), or an infected blood vessel graft. Persistent bacteremia can also occur as part of the infection process of typhoid fever, brucellosis, and bacterial meningitis. Left untreated, conditions causing persistent bacteremia can be potentially fatal.

Bacteremia is clinically distinct from sepsis, which is a condition where the blood stream infection is associated with an inflammatory response from the body, often causing abnormalities in body temperature, heart rate, breathing rate, blood pressure, and white blood cell count.

Multiple drug resistance (MDR), multidrug resistance or multiresistance is antimicrobial resistance shown by a species of microorganism to multiple antimicrobial drugs. The types most threatening to public health are MDR bacteria that resist multiple antibiotics; other types include MDR viruses, fungi, and parasites (resistant to multiple antifungal, antiviral, and antiparasitic drugs of a wide chemical variety). Recognizing different degrees of MDR, the terms extensively drug resistant (XDR) and pandrug-resistant (PDR) have been introduced. The definitions were published in 2011 in the journal "Clinical Microbiology and Infection" and are openly accessible.

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.

Bacteremia (also bacteraemia) is the presence of bacteria in the blood. Blood is normally a sterile environment, so the detection of bacteria in the blood (most commonly accomplished by blood cultures) is always abnormal. It is distinct from sepsis, which is the host response to the bacteria.

Bacteria can enter the bloodstream as a severe complication of infections (like pneumonia or meningitis), during surgery (especially when involving mucous membranes such as the gastrointestinal tract), or due to catheters and other foreign bodies entering the arteries or veins (including during intravenous drug abuse). Transient bacteremia can result after dental procedures or brushing of teeth.

Bacteremia can have several important health consequences. The immune response to the bacteria can cause sepsis and septic shock, which has a high mortality rate. Bacteria can also spread via the blood to other parts of the body (which is called hematogenous spread), causing infections away from the original site of infection, such as endocarditis or osteomyelitis. Treatment for bacteremia is with antibiotics, and prevention with antibiotic prophylaxis can be given in high risk situations.

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

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

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

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

In most cases AIT is characterized by onset of pain, firmness, tenderness, redness or swelling in the anterior aspect of the neck. Patients will also present with a sudden fever, dysphagia and dysphonia. Symptoms may be present from 1 to 180 days, with most symptoms lasting an average of about 18 days. The main issue associated with the diagnosis of AIT is differentiating it from other more commonly seen forms of thyroid conditions. Pain, fever and swelling are often much more severe and continue to get worse in patients who have AIT compared to those with other thyroid conditions. Patients who are suspected of having AIT often undergo tests to detect for elevated levels of white blood cells as well as an ultrasound to reveal unilobular swelling. Depending on the age and immune status of the patient more invasive procedures may be performed such as fine needle aspiration of the neck mass to facilitate a diagnosis.

In cases where the infection is thought to be associated with a sinus fistula it is often necessary to confirm the presence of the fistula through surgery or laryngoscopic examination. While invasive procedures can often tell definitively whether or not a fistula is present, new studies are working on the use of computed tomography as a useful method to visualize and detect the presence of a sinus fistula.

Pneumonia can cause respiratory failure by triggering acute respiratory distress syndrome (ARDS), which results from a combination of infection and inflammatory response. The lungs quickly fill with fluid and become stiff. This stiffness, combined with severe difficulties extracting oxygen due to the alveolar fluid, may require long periods of mechanical ventilation for survival.

Sepsis is a potential complication of pneumonia but occurs usually in people with poor immunity or hyposplenism. The organisms most commonly involved are "Streptococcus pneumoniae", "Haemophilus influenzae", and "Klebsiella pneumoniae". Other causes of the symptoms should be considered such as a myocardial infarction or a pulmonary embolism.

Acute infectious thyroiditis (AIT) also known as suppurative thyroiditis, microbial inflammatory thyroiditis, pyrogenic thyroiditis and bacterial thyroiditis.

The thyroid is normally very resistant to infection. Due to a relatively high amount of iodine in the tissue, as well as high vascularity and lymphatic drainage to the region, it is difficult for pathogens to infect the thyroid tissue. Despite all this, a persistent fistula from the piriform sinus may make the left lobe of the thyroid susceptible to infection and abscess formation. AIT is most often caused by a bacterial infection but can also be caused by a fungal or parasitic infection, most commonly in an immunocompromised host.

Carbapenem-resistant Enterobacteriaceae (CRE) have been defined as carbapenem-nonsusceptible and extended-spectrum cephalosporin-resistant "Escherichia coli, Enterobacter aerogenes, Enterobacter cloacae" complex, "Klebsiella pneumoniae", or "Klebsiella oxytoca". Some exclude ertapenem resistance from the definition.

With treatment, most types of bacterial pneumonia will stabilize in 3–6 days. It often takes a few weeks before most symptoms resolve. X-ray finding typically clear within four weeks and mortality is low (less than 1%). In the elderly or people with other lung problems, recovery may take more than 12 weeks. In persons requiring hospitalization, mortality may be as high as 10%, and in those requiring intensive care it may reach 30–50%. Pneumonia is the most common hospital-acquired infection that causes death. Before the advent of antibiotics, mortality was typically 30% in those that were hospitalized.

Complications may occur in particular in the elderly and those with underlying health problems. This may include, among others: empyema, lung abscess, bronchiolitis obliterans, acute respiratory distress syndrome, sepsis, and worsening of underlying health problems.

Carbapenem-resistant Enterobacteriaceae (CRE) or carbapenemase-producing Enterobacteriaceae (CPE) are Gram-negative bacteria that are resistant to the carbapenem class of antibiotics, considered the drugs of last resort for such infections. They are resistant because they produce an enzyme called a carbapenemase that disables the drug molecule. The resistance can vary from moderate to severe. Enterobacteriaceae are common commensals and infectious agents. Experts fear CRE as the new "superbug". The bacteria can kill up to half of patients who get bloodstream infections. Tom Frieden, former head of the Centers for Disease Control and Prevention has referred to CRE as "nightmare bacteria". Types of CRE are sometimes known as KPC (Klebsiella pneumoniae carbapenemase) and NDM (New Delhi Metallo-beta-lactamase). KPC and NDM are enzymes that break down carbapenems and make them ineffective. Both of these enzymes, as well as the enzyme VIM (Verona Integron-Mediated Metallo-β-lactamase) have also been reported in Pseudomonas.

An overwhelming post-splenectomy infection (OPSI) or Overwhelming post-splenectomy sepsis (OPSS) is a rare but rapidly fatal infection occurring in individuals following removal of the spleen. The infections are typically characterized by either meningitis or sepsis, and are caused by encapsulated organisms including "Streptococcus pneumoniae".

The risk of OPSI is 0.23–0.42 percent per year, with a lifetime risk of 5 percent. Most infections occur in the first few years following splenectomy, but the risk of OPSI is lifelong. OPSI is almost always fatal without treatment, and modern treatment has decreased the mortality to approximately 40–70 percent. Individuals with OPSI are most commonly treated with antibiotics and supportive care. Measures to prevent OPSI include vaccination and prophylactic antibiotics.

Indwelling catheters have recently been identified with hospital acquired infections. Procedures using Intravascular Antimicrobial Lock Therapy can reduce infections that are unexposed to blood-borne antibiotics. Introducing antibiotics, including ethanol, into the catheter (without flushing it into the bloodstream) reduces the formation of biofilms.

Contact transmission is divided into two subgroups: direct-contact transmission and indirect-contact transmission.

An airborne disease is any disease that is caused by pathogens that can be transmitted through the air. Such diseases include many of considerable importance both in human and veterinary medicine. The relevant pathogens may be viruses, bacteria, or fungi, and they may be spread through breathing, talking, coughing, sneezing, raising of dust, spraying of liquids, toilet flushing or any activities which generates aerosol particles or droplets. Human airborne diseases do not include conditions caused by air pollution such as volatile organic compounds (VOCs), gasses and any airborne particles, though their study and prevention may help inform the science of airborne disease transmission.

People usually complain of intense pain that may seem excessive given the external appearance of the skin. People initially have signs of inflammation, fever and a fast heart rate. With progression of the disease, often within hours, tissue becomes progressively swollen, the skin becomes discolored and develops blisters. Crepitus may be present and there may be a discharge of fluid, said to resemble "dish-water". Diarrhea and vomiting are also common symptoms.

In the early stages, signs of inflammation may not be apparent if the bacteria are deep within the tissue. If they are "not" deep, signs of inflammation, such as redness and swollen or hot skin, develop very quickly. Skin color may progress to violet, and blisters may form, with subsequent necrosis (death) of the subcutaneous tissues.

Furthermore, people with necrotizing fasciitis typically have a fever and appear sick. Mortality rates are as high as 73% if left untreated. Without surgery and medical assistance, such as antibiotics, the infection will rapidly progress and will eventually lead to death.

When it affects the groin it is known as Fournier gangrene.

Antimicrobial resistance (AMR) is the ability of a microbe to resist the effects of medication previously used to treat them. The term includes the more specific "antibiotic resistance", which applies only to bacteria becoming resistant to antibiotics. Resistant microbes are more difficult to treat, requiring alternative medications or higher doses, both of which may be more expensive or more toxic. Microbes resistant to multiple antimicrobials are called multidrug resistant (MDR); or sometimes superbugs.

Resistance arises through one of three mechanisms: natural resistance in certain types of bacteria, genetic mutation, or by one species acquiring resistance from another. All classes of microbes can develop resistance: fungi develop antifungal resistance, viruses develop antiviral resistance, protozoa develop antiprotozoal resistance, and bacteria develop antibiotic resistance. Resistance can appear spontaneously because of random mutations; or more commonly following gradual buildup over time.

Preventive measures include only using antibiotics when needed, thereby stopping misuse of antibiotics or antimicrobials. Narrow-spectrum antibiotics are preferred over broad-spectrum antibiotics when possible, as effectively and accurately targeting specific organisms is less likely to cause resistance. For people who take these medications at home, education about proper use is essential. Health care providers can minimize spread of resistant infections by use of proper sanitation and hygiene, including handwashing and disinfecting between patients, and should encourage the same of the patient, visitors, and family members.

Rising drug resistance is caused mainly by use of antimicrobials in humans and other animals, and spread of resistant strains between the two. Antibiotics increase selective pressure in bacterial populations, causing vulnerable bacteria to die; this increases the percentage of resistant bacteria which continue growing. With resistance to antibiotics becoming more common there is greater need for alternative treatments. Calls for new antibiotic therapies have been issued, but new drug development is becoming rarer.

Antimicrobial resistance is on the rise. Estimates are that 700,000 to several million deaths result per year. Each year in the United States, at least 2 million people become infected with bacteria that are resistant to antibiotics and at least 23,000 people die as a result. There are public calls for global collective action to address the threat include proposals for international treaties on antimicrobial resistance. Worldwide antibiotic resistance is not fully mapped, but poorer countries with weak healthcare systems are more affected.