Horizontal transmission of infection

Horizontal transmission. With horizontal transmission, infective microbes are usually shared from an infected host before they are transmitted to a new one. Contact transmission is the most frequent mode of transmission, including healthcare associated infections. Direct contact transmission involves body surface to body surface contact with the concomitant transfer of microorganisms from an infected host to an uninfected recipient. Example includes - Staphylococci such as MRSA. Indirect contact transmission usually involves contact between a susceptible host and a contaminated inanimate object, such as equipment, or instruments and environmental surfaces. This is usually as a direct result of unwashed hands that can contaminate objects, or the environment.

Infective microorganisms that stimulate the activities of the host can influence their transmission rate ,for example irritation of the respiratory tract, causing coughing or sneezing from the host can enhance the transmission of microorganisms. Droplet transmission can be considered a form of contact transmission when it lands on surfaces infecting them. The microorganisms contained in droplets generated from the source person during coughing, sneezing, or talking can be transmitted a short distance through the air, usually less than a meter, before contaminating the new host in their nasal mucosa or their mouth but droplets do not remain suspended in the air.

This is unlike airborne transmission where the microorganisms are contained and disseminated on very small airborne droplet nuclei that are usually less than five millimetres. These little droplet nuclei can remain suspended in the air for long periods of time and can be dispersed by air currents and may be inhaled by the susceptible host. Transmission includes Mycobacterium tuberculosis, Rubeola and even varicella, or chickenpox. On the other hand, common vehicle transmission applies to microorganisms transmitted by contaminated items such as food, water, and medication, and infection is spread to multiple hosts. Of course, this can cause explosive outbreaks - cholera and typhoid are easily spread through this route, especially in countries, or in situations, where drinking water becomes contaminated with untreated sewage.

Microbes increase their ability to survive in the external environment if they are resistant to drying out ,and to thermal inactivation. Finally, some microbes are transferred from one host to another by a vector such as insects - mosquitoes are an example. These can extract microbes from an infected body before actively placing them within a new host, again causing infection. Now, transmission depends on many factors, including the number of microorganisms shed by the host, the number of microbes required to infect a new host, as well as the stability of the microbe within an environment before it causes a new infection. There we have it - horizontal transmission.