Direct Host-to-Host Transmission
Host-to-host transmission occurs when an infected host transmits a disease directly to a susceptible host without the assistance of an intermediate host or inanimate object. Upper
respiratory infections such as the common cold and influenza are most often transmitted host to host by droplets resulting from sneezing or coughing. Many of these droplets, however, do not remain airborne for long. Transmission, therefore, requires close, although not necessarily intimate, person-to-person contact.
Some pathogens are extremely sensitive to environmental factors such as drying and heat and are unable to survive for significant periods of time away from the host.
Direct contact also transmits skin pathogens such as staphylococci (boils and pimples) and fungi (ringworm). These pathogens often spread by indirect means as well because they are relatively resistant to environmental conditions such as drying.
Indirect Host-to-Host Transmission
Indirect transmission of an infectious agent can be facilitated by either living or inanimate carriers. Living carriers transmitting
pathogens are called vectors. Commonly, arthropods (mites, ticks, or fleas) or vertebrates (dogs, cats, or rodents) act as vectors. Arthropod vectors may not be hosts for the pathogen, but may carry the agent from one host to another. Many arthropods obtain their nourishment by biting and sucking blood, and if the pathogen is present in the blood, the arthropod vector may ingest the pathogen and transmit it when biting another individual. In some cases viral pathogens replicate in the arthropod vector, which is then considered an alternate host. Such is the case for
West Nile virus. Such replication leads to an
increase in pathogen numbers, increasing the probability that a subsequent bite will lead to infection.
Direct Host-to-Host Transmission
Host-to-host transmission occurs when an infected host transmits a disease directly to a susceptible host without the assistance of an intermediate host or inanimate object. Upper
respiratory infections such as the common cold and influenza are most often transmitted host to host by droplets resulting from sneezing or coughing. Many of these droplets, however, do not remain airborne for long. Transmission, therefore, requires close, although not necessarily intimate, person-to-person contact.
Some pathogens are extremely sensitive to environmental factors such as drying and heat and are unable to survive for significant periods of time away from the host.
Direct contact also transmits skin pathogens such as staphylococci (boils and pimples) and fungi (ringworm). These pathogens often spread by indirect means as well because they are relatively resistant to environmental conditions such as drying.
Indirect Host-to-Host Transmission
Indirect transmission of an infectious agent can be facilitated by either living or inanimate carriers. Living carriers transmitting
pathogens are called vectors. Commonly, arthropods (mites, ticks, or fleas) or vertebrates (dogs, cats, or rodents) act as vectors. Arthropod vectors may not be hosts for the pathogen, but may carry the agent from one host to another. Many arthropods obtain their nourishment by biting and sucking blood, and if the pathogen is present in the blood, the arthropod vector may ingest the pathogen and transmit it when biting another individual. In some cases viral pathogens replicate in the arthropod vector, which is then considered an alternate host. Such is the case for
West Nile virus. Such replication leads to an
increase in pathogen numbers, increasing the probability that a subsequent bite will lead to infection.
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