Infections

Definition

Infection is defined as the invasion of the body by a pathogen, which is any microorganism that causes disease. As a reaction to the invasion, various signs and symptoms develop in the infected individual.

Types of Infection

Infections can be categorized into two types: localized infections, which are limited to a small area, and systemic infections, which are widespread throughout the body and often disseminated via the bloodstream.

Agents of Infectious Diseases

Infectious diseases are caused by pathogens, which include bacteria, viruses, fungi, and parasites. The structural differences among these agents influence their transmission, the harm they may cause, and the methods used for treatment.

Viruses

Viruses are unique in that they are not considered alive; they are non-cellular and cannot reproduce independently. A typical virus comprises nucleic acid, a protein coat known as a capsid, and sometimes a membrane envelope. Examples of viral infections include HIV, Herpes, H1N1, SARS, MERS-CoV, and Covid-19.

Bacteria

Bacteria are characterized by their requirement for no living tissue to survive, demonstrating adaptability, but with specific growth parameters. They can be classified as aerobes, which require oxygen, or anaerobes, which cannot survive in its presence. Bacteria possess all the necessary cellular mechanisms for self-replication and while many bacteria are beneficial, some are pathogenic. The cell wall of bacteria provides protection and is made of peptidoglycan. There are two primary types of bacteria: Gram-positive, which have thick walls with a simple structure, and Gram-negative, which possess thinner walls, a more complex structure, and are often more pathogenic.

Fungi

Fungi are free-living, eukaryotic organisms that secrete enzymes for external digestion and absorb nutrients. They can be categorized into molds, which are rapidly growing fungi that reproduce asexually via spores, and yeasts, which are single-celled fungi that reproduce asexually through cell division or budding; for example, Candida causes Thrush.

Parasites

Parasites obtain nutrition from a living host and can be divided into several types: protozoa, which are unicellular eukaryotes (e.g., Malaria); helminths, which are worm-like parasites (e.g., Tapeworm, trichinosis); and arthropods, including vectors of infection like ticks and mosquitoes. Ectoparasites infect external body surfaces, such as mites, lice, and fleas.

Infectious Diseases Terminology

In the context of infectious diseases, the host typically refers to humans. Infection is distinguished from colonization; infection refers to the presence and multiplication of pathogens, whereas colonization indicates establishing a presence without causing disease, such as seen in tracheostomies.

Microflora and commensalism are terms that describe the relationship of microorganisms benefiting from humans without causing harm, for example, Candida in the mouth and gastrointestinal tract. Mutualism is a beneficial interaction, such as the production of vitamin K in the human GI tract.

Terms for Infection and Damage

Terms associated with infection include -itis, which indicates inflammation and may not necessarily be due to infection, and -emia, which indicates the presence of pathogens in the blood. Sepsis refers to a bloodstream infection, while virulence describes a pathogen's capacity to cause severe disease. Opportunistic infections arise from pathogens in hosts with weakened immune systems.

Modes of Transmission

Infectious agents can be transmitted through various modes, including penetration (direct access via blood, such as needle injuries), direct contact (skin-to-skin contact or bites and stings from animals), ingestion (consuming contaminated food or drink), and inhalation (breathing in infectious agents).

Source of Disease Transmission

The source of disease transmission refers to the location or substance from which the infectious agent is acquired. Transmission occurs through various pathways, including the source, portal of entry, and vector. A host is an organism that supports the growth of another organism, exemplified by humans hosting various bacterial infections.

Types of Infections

Infections can be classified as nosocomial, which are acquired in hospital settings, or community-acquired, which are infections contracted outside of hospitals.

Chain of Infection

Host resistance influences the chain of infection and is determined by the individual's general health, pre-existing conditions, age, and the virulence of the infectious agent.

Mechanisms of Disease Production

Virulence factors play a significant role in disease production. These include toxins, such as endotoxins like LPS from gram-negative bacteria, and exotoxins, such as those responsible for botulism and tetanus. Pathogens utilize adhesion factors to attach to and colonize the host, while evasive factors prevent their destruction by the host, and invasive factors are enzymes that facilitate penetration into host tissues.

Clinical Manifestations of Infection

Infections exhibit both specific symptoms (e.g., respiratory distress in children) and nonspecific symptoms (e.g., fever, malaise, fatigue, weakness). The disease course in an infection typically includes several stages: an incubation period characterized by active replication without symptoms, a prodromal stage with initial mild symptoms, an acute stage where rapid pathogen spread occurs, followed by the convalescent period where the pathogen is eliminated and symptoms resolve, ultimately leading to complete resolution where the pathogen is entirely gone.

Exceptions to Disease Course

Certain infections may have an irregular disease course, such as chronic infections, which may not go through a convalescent phase, or subclinical infections, which resolve without noticeable symptoms. Fulminant infections exhibit an abrupt onset with no prodromal phase, while fatal infections show no resolution or convalescence.

Diagnostic Tests for Infections

Diagnostic tests are crucial for determining the presence of infections. Differential diagnosis helps identify diseases based on clinical data. Common diagnostic tests include cultures, serology, DNA/RNA sequencing, blood panels, and diagnostic imaging.

Signs of Bacterial Infection

Bacterial infections manifest local signs such as swelling, erythema, and pain, alongside systemic signs like fever, leukocytosis, and fatigue. The cellular response to infection typically reveals increased neutrophils during bacterial infections, increased eosinophils in response to parasites, and a potential decrease in neutrophils during viral infections.

Treatment of Infections

The treatment of infections includes various antimicrobial agents: antibacterial agents target cell wall and protein synthesis, antiviral medications focus on viral DNA/RNA synthesis, and antifungal treatments affect fungal cell membranes. Antibiotic resistance can occur through mechanisms such as enzymatic inactivation of antibiotics, genetic mutations that alter binding sites, the bypassing of pathways around antibiotics, and changes to the cell wall that prevent antibiotic access. Among antifungals, the polyene family binds to ergosterol in fungal membranes, while the imidazole class inhibits ergosterol synthesis.