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Why can a person be infected without developing an infectious disease?
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infectious disease
Birds are the primary reservoirs of West Nile virus. The virus is transmitted to humans and horses by mosquitoes that have bitten infected birds.
It is important to distinguish between infection and disease. An infection occurs when microorganisms, such as bacteria, viruses, or parasites, enter and colonize the body. A disease, however, only develops if those agents cause significant tissue injury or disrupt normal bodily functions.12
A person may be infected without developing a disease for several reasons:
- Nonpathogenic Agents: Many microorganisms are commensal, meaning they live on or in the human body without causing harm under normal conditions.1
- Host Defenses: The body’s immune system often successfully contains or eliminates invaders before they can cause damage. Physical barriers like the skin, along with innate immunity and acquired immunity, work to neutralize pathogens.34
- Asymptomatic Carriers: Some individuals, known as asymptomatic carriers, harbor and can transmit infectious agents without ever manifesting symptoms of illness. In these cases, the host may have sufficient resistance to prevent the agent from causing clinical disease, even while the agent continues to replicate and potentially spread to others.563
Ultimately, for an infection to progress to disease, the agent must overcome host defenses, survive the immune response, and possess enough virulence to cause meaningful injury.1