As discussed previously, humans are constantly exposed to a multitude of microorganisms in the forms of bacteria, viruses, parasites, or fungi. Although many microorganisms do not pose a health threat to humans, some microorganisms cause human disease and are known as pathogens. Typically, an individual’s immune system is able to rid the body of pathogens without developing an infection. An infection occurs when a susceptible host is invaded by a pathogen that multiplies and causes disease.

Bacterial Infections

In this section, we will review common bacterial infections that nurses will see in the hospital and in the community. Bacteria are one-celled organisms without a true nucleus or cellular organelles. They synthesize deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and proteins, and can reproduce independently, but they require a host for a suitable environment for multiplication. Bacteria cause cellular injury by releasing toxins that are either exotoxins (enzymes released by gram-positive bacteria into the host) or endotoxins (part of the bacterial cell wall of gram-negative bacteria that can cause damage to the host even if the bacteria are dead). Diseases caused by bacterial invasion depend on the type of bacterial pathogen and the area of the body that is primarily invaded.^[1]

General symptoms of bacterial infections

Individuals of any age can develop a bacterial infection. Bacteria can infect any area of the body, including the skin, bladder, lungs, intestines, brain, and more. A bacterial infection can also spread throughout the blood, causing a condition described as sepsis.

Some generalized symptoms of infection include fever, chills, and fatigue.  Localized symptoms of infection can include swelling, pain at the site, redness and organ dysfunction.

Common bacterial infections

1. Salmonella is a type of infection often described as food poisoning. It causes severe stomach cramps, fever, diarrhea, and vomiting. Salmonella is caused by a non-typhoidal salmonellae bacteria found in the intestinal tracts of humans and other animals, and the most recognized method of infection is through undercooked poultry.  Salmonella strains sometimes cause infection in urine, blood, bones, joints, or the nervous system (spinal fluid and brain), and can cause severe disease.
2. Escherichia coli (E. coli) causes gastrointestinal (GI) distress. The infection usually resolves on its own, but if it affects the kidney it can be severe or even fatal. E. Coli bacteria commonly spread through contaminated food (typically meat products, but sometimes even via uncooked vegetables).
3. Tuberculosis (TB) is a highly contagious disease caused by the Mycobacterium tuberculosis bacteria. It most commonly causes a lung infection, and it rarely affects the brain.  Common symptoms of TB include cough, chest pain, weakness, weight loss, fever and lack of appetite. Although the incidence of active TB disease in the overall Canadian population has been decreasing over time and is among the lowest in the world, high rates persist among Aboriginal peoples and among foreign-born individuals.
4. Methicillin-resistant Staphylococcus aureus (MRSA) is an antibiotic-resistant bacteria that can be deadly, particularly in people who have compromised immune systems. Staphylococcus bacteria normally live on the skin and in the nose, usually without causing problems. But if these bacteria become resistant to antibiotics, they can cause serious infections, especially in people who are ill or weak. Symptoms of MRSA depend on where the infection is – for example, if MRSA is causing an infection in a wound, that area of the skin may have purulent drainage, and may be red, tender and warm to touch.  MRSA is different from other types of staphylococcus because it cannot be treated with certain antibiotics, such as methicillin, making treatment complex and prolonging hospitalization.  MRSA can be spread by contact with contaminated persons or through contaminated objects. It is a common hospital-acquired infection.
5. Clostridium difficile (C. diff) is a bacteria normally found in the intestine. For most healthy people, C.diff is not a health risk. It can cause GI illness when it overgrows due to antibiotic use or an impaired immune system. C.diff is the most frequent cause of infectious diarrhea in hospitals and health care facilities. Symptoms include watery diarrhea, fever, loss of appetite, nausea, and abdominal pain/tenderness. It can spread from person to person through the faecal-oral route; for example, if a health care provider does not clean their hands with soap and water after caring for a client with C. diff, they can potentially pass on the infection to other people they touch.  It is important to note that c. diff bacterial spores cannot be killed with alcohol hand sanitizer, therefore infected persons and their care providers must always wash using soap and water, and practice frequent hand hygiene.
6. Bacterial pneumonia is a lung infection that can be caused by an array of different bacteria, including Streptococcus pneumonia (main culprit), Haemophilus Influenzae, Staphlyococcus aureus and others. Note that some pneumonia are caused by viruses and/or fungi, but bacterial pneumonias are the most common. These infections are typically spread through air particles from coughing or sneezing (droplets) and aspiration, which then leads to an infection in the respiratory tract.  Pneumonia can be further divided into community-acquired, hospital-acquired (HAP) and ventilator-associated (VAP).  Community-acquired bacterial pneumonia is less likely to involve multi-drug resistant bacteria, so is therefore usually less severe compared to HAP and VAP.
7. Heliobacter pylori (H. pylori) is a type of bacteria associated with stomach ulcers and chronic gastritis. The environment of the GI system can change due to reflux, acidity, and smoking, which predisposes individuals to this bacterial infection. H. pylori exists in the stomachs of approximately 50% of the population, typically causing no harm. Issues arise when H. pylori are able to adhere to the cells of the stomach, and begin to disrupt cell function, causing inflammation and ulcer formation.  Symptoms of H. pylori infection include abdominal pain, loss of appetite, nausea, and bloating. In some cases, H. pylori infection can lead to duodenal ulcers as well.

Sepsis

If unmanaged, bacterial infections can lead to a state of severe disease, called sepsis.  Sepsis is a life-threatening condition as it can result in damage to body tissues and organs.  Severe sepsis can lead to low blood pressure, and therefore poor blood flow and organ dysfunction.  If sepsis persists, worsens and does not improve with fluid replacement, septic shock can occur.  In order to gain a better understanding of Sepsis and Septic Shock, review the linked Khan Academy resources.

Viral infections

A virus is a pathogen with nucleic acid molecule within a protein coat.  It requires a living host for replication. An invading virus may immediately cause disease or may remain relatively dormant for years.  Diseases develop as a result of interference of normal cellular functioning of the host, with the destruction of the virus by the immune system also requiring death of the host cell.

Common viral infections:

1. Human immunodeficiency virus (HIV)
2. Hepatitis A, B, C, or E
3. Human papillomavirus (HPV)
4. Respiratory syncytial virus (RSV)
5. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)

Fungal Infections

A fungus is any spore-producing microorganism, which includes yeasts, molds, and mushrooms. Fungi typically grow and proliferate in moist areas of the body, such as between the toes, in the groin, under the panniculus, and under the breasts. In an otherwise healthy individual, fungi do not cause disease and are contained by the body’s natural flora.  In the immunocompromised individual, they can result in infections that lead to death.  Obese people and people with diabetes are also more susceptible to fungal infections.

Common fungal infections:

1. Tinea pedis (athlete’s foot):  this infection can occur in healthy individuals as well
2. Candidiasis (yeast infection)
3. Aspergillosis (lung infection caused by mold)
4. Histoplasmosis (fungal lung infection)

Parasitic Infections

Parasites or protozoa generally infect individuals with compromised immune responses. They are typically found in dead material in water and soil and are spread by the fecal–oral route by ingesting food or water that is contaminated with the parasitic spores or cysts. Disease may develop in an otherwise healthy individual when the spores invade organs and stimulate an immune response, interfering with normal functioning of the organ system.

Common parasitic infections:

1. Giardiasis
2. Cryptosporidium
3. Toxoplasmosis
4. Malaria

In the next chapters, we will review assessment and management for infection, focusing primarily on bacterial infections.