Infection and Infectious Disease. Interactions between Host and Microorganisms. Forms of Infection.
- Infection and Infectious Disease
- Infection is defined as the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to these agents and the toxins they produce.
- It is a biological process where microorganisms (bacteria, viruses, fungi) penetrate, multiply, and spread within the body.
- An infection may or may not lead to an infectious disease.
- Infectious disease, also known as transmissible or communicable disease, is an illness resulting from an infection.
- It is characterised by clinical symptoms such as fever or cough.
- Infectious diseases are caused by bacteria, fungi, viruses, or parasites. Definitive proof of the cause can be demonstrated by fulfilling the Henle-Koch postulates, though modified forms are sometimes applied, especially for infections caused by subcellular entities.
- Interactions between Host and Microorganisms
- The genesis, clinical picture, and outcome of an infection involve complex relationships between the host and invading organisms.
- The pathogen enters the body, must evade or overcome the host's defenses, and potentially causes disease.
- This interaction and the resulting disease are dependent on several factors:
- Pathogen properties: including pathogenicity (the capacity of a pathogen species to cause disease) and virulence (the sum of the disease-causing properties of a strain of a pathogenic species). Virulence can be influenced by factors like adhesins, invasins, impedins, aggressins (toxins, enzymes), and modulins.
- Host immune response: The host's immune system plays a significant role in controlling infections. This includes innate immunity (e.g., macrophages, neutrophils, physical barriers like skin) and adaptive immunity (e.g., T-cells, B-cells, antibodies, cytotoxic cells). Excessive
host immune responses can contribute to the clinical symptoms of a bacterial infection.
- Host factors: A compromised immune system (e.g., in HIV/AIDS, diabetes, immunosuppressive treatments) makes the host more vulnerable.
- Environmental conditions.
- Some microorganisms are commensals or normal inhabitants of the skin and mucosa, collectively forming the normal flora or microbiota. Bacteria from normal microbiota are typically harmless, but some can cause infections in individuals with impaired defense mechanisms (immunocompromised).
- Opportunistic microorganisms (or facultatively pathogenic microorganisms) may exhibit pathogenicity in individuals with weakened defense responses. They can cause disease under circumstances where the host's resistance is low, or if introduced into anatomical sites where they are not normally found. Disruptions to the normal microbiota balance (dysbiosis) can also lead to opportunistic infections by otherwise harmless species.
- Pathogenic microorganisms (primary pathogens) are not typically part of the human microbiota and cause disease upon infection.