Infection- Role of Microorganisms in the Infectious Process. Properties of Infectious Causative Agents Infectivity, Pathogenicity, Virulence, Contagiousness. Pathogenicity and Virulence Factors. Role of the Host in the Development of the Infections.
Role of Microorganisms in the Infectious Process The infectious process begins when a pathogen comes into contact with a suitable host. Not all encounters lead to infection; the host must be susceptible to the specific pathogen.
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 defences, and potentially causes disease.
Properties of Infectious Causative Agents
- Infectivity: This is the ability of a microbe to establish infection in a host, regardless of whether symptoms appear. Some microbes can infect multiple species, while others are species-specific. The infectious dose, which is the minimum number of microbes required to cause infection, varies between pathogens. A low infectious dose means only a small number of pathogens are needed (e.g., Shigella spp.), while a high infectious dose requires a larger number to overwhelm host defences (e.g., cholera). The infectious dose is influenced by the pathogen's virulence, the host's immune status, and the route of transmission.
- Pathogenicity: This refers to the ability of a microbe to cause disease by overcoming host defences. Pathogenicity can be used to characterise a particular species. Some infections can be asymptomatic or subclinical, meaning they do not cause visible symptoms but may still elicit an immune response. Other microbes typically result in clinical disease with symptoms. Pathogenic bacteria (primary pathogens) cause disease upon infection and are not typically associated with the host. Obligatory pathogenic bacteria have mechanisms to promote their growth at the expense of the host's tissue or organ function, and disease results from damage or loss of tissue/organ function or development of host inflammatory response.
- Virulence: This term describes the severity of a disease caused by a pathogen. It is the sum of the disease-causing properties of a strain of a pathogenic species. Virulence factors contribute to a bacterium's ability to invade the host, cause disease, and evade host defences. Viral virulence and the implantation point can impact the severity of the disease.