Transmissible disease

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected. In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected. The term strictly refers to the transmission of microorganisms directly from one individual to another by one or more of the following means: Transmission can also be indirect, via another organism, either a vector (e.g. a mosquito or fly) or an intermediate host (e.g. tapeworm in pigs can be transmitted to humans who ingest improperly cooked pork). Indirect transmission could involve zoonoses or, more typically, larger pathogens like macroparasites with more complex life cycles. Transmissions can be autochthonous (i.e. between two individuals in the same place) or may involve travel of the microorganism or the affected hosts. An infectious disease agent can be transmitted in two ways: as horizontal disease agent transmission from one individual to another in the same generation (peers in the same age group) by either direct contact (licking, touching, biting), or indirect contact through air – cough or sneeze (vectors or fomites that allow the transmission of the agent causing the disease without physical contact)or by vertical disease transmission, passing the agent causing the disease from parent to offspring, such as in prenatal or perinatal transmission. The term infectivity describes the ability of an organism to enter, survive and multiply in the host, while the infectiousness of a disease agent indicates the comparative ease with which the disease agent is transmitted to other hosts. Transmission of pathogen can occur in various ways including physical contact, contaminated food, body fluids, objects, airborne inhalation, or through vector organisms. The route of transmission is important to epidemiologists because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as cholera. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that polio is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route. 'Airborne transmission refers to infectious agents that are spread via droplet nuclei (residue from evaporated droplets) containing infective microorganisms. These organisms can survive outside the body and remain suspended in the air for long periods of time. They infect others via the upper and lower respiratory tracts.' Diseases that are commonly spread by coughing or sneezing include bacterial meningitis, chickenpox, common cold, influenza, mumps, strep throat, tuberculosis, measles, rubella, whooping cough, SARS and leprosy. 'Droplet transmission occurs when respiratory droplets generated via coughing, sneezing or talking contact susceptible mucosal surfaces, such as the eyes, nose or mouth. Transmission may also occur indirectly via contact with contaminated formites with hands and then mucosal surfaces. Respiratory droplets are large and are not able to remain suspended in the air thus they are usually dispersed over short distances.' The pathogen-containing particles, also called Flügge droplets (after Carl Flügge), are 0,1–2 mm in diameter, and are reduced by evaporation to droplet nuclei – small (smaller than 100 μ in diameter), dry particles that can remain airborne for long periods.