Transmission of infections





The transmission of disease-causing pathogens, 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 transmission refers to the transmission of microorganisms directly from one individual to another by one or more of the following means:



airborne transmission 



droplet transmission 



direct physical contact 



indirect physical contact 



fecal–oral transmission 



Indirect by vectors, intermediate hosts, or zoonoses



Airborne transmission refers to very small dry and wet particles that stay in the air for long periods of time allowing airborne contamination even after the departure of the host. 



In airborne transmission particle size < 5 μm.



Droplet transmission refers to small and usually wet particles that stay in the air for a short period of time. 



Contamination from droplet transmission usually occurs in the presence of the host. 



Droplet transmission particle size is > 5 μm.



Direct physical contact, by touching an infected individual, including sexual contact.



indirect physical contact, usually by touching a contaminated surface, including soil.



fecal–oral transmission, usually from unwashed hands, contaminated food or water sources due to lack of sanitation and hygiene, an important transmission route in pediatrics, veterinary medicine and developing countries.



Transmission be indirect by organism, either a vector , like a mosquito or fly or an intermediate host, like  a tapeworm in pigs can be transmitted to humans who ingest improperly cooked pork.



Indirect transmission could involve zoonoses or, more macroparasites with more complex life cycles. 



An infectious disease agent can be transmitted in two ways: as horizontal disease agent transmission from one individual to another, either direct contact or indirect contact, or by vertical disease transmission, passing the agent causing the disease from parent to offspring, such as in prenatal or perinatal transmission.



Infectivity describes the ability of an organism to enter, survive and multiply in the host.



Then infectiousness of a disease agent compares  the ease with which the disease agent is transmitted to other hosts.



Transmission of pathogens can occur by direct contact, through contaminated food, body fluids or objects, by airborne inhalation or through vector organisms.



Community transmission means that the source of infection for the spread of an illness is unknown or a link in terms of contacts between patients and other people is missing: difficulty in understanding the epidemiological link in the community beyond confirmed cases.



Patterns of contact vary between different populations.



Patterns of contact differ among groups of populations depending on socio-economic, cultural and other features: 


Poor personal and food hygiene may result in increased transmission of diseases by the fecal-oral route, such as cholera, and polio.



Two routes are considered to be airborne: Airborne infections and droplet infections.



Airborne transmission refers to infectious agents that are spread via residue from evaporated droplets that contain infective microorganisms. 



These airborne organisms can survive outside the body and remain suspended in the air for long periods of time. 



Airborne transmitted organisms 


infect others via the upper and lower respiratory tracts.



The size of the particles for airborne infections need to be < 5 μm.



Airborne organisms are included in  both dry and wet aerosols and thus requires usually higher levels of isolation since it can stay suspended in the air for longer periods of time. 



With airborne transmission separate ventilation systems or negative pressure environments are needed to avoid general contamination: tuberculosis, chickenpox, measles.



Respiratory droplet transmission are released through talking, coughing, or sneezing, and is a common form of infection transmission.



Respiratory droplet transmission is the usual route for respiratory infections. 



Transmission can occur when respiratory droplets reach susceptible mucosal surfaces, such as in the eyes, nose or mouth. 



The infectivity, and dispersal and removal of viruses are subject to environmental factors: temperature, humidity, and ventilation that play a role in the persistence of pathogens.



Environmental factors such as temperature, humidity, and ventilation play a role in the persistence, infectivity, and dispersal and removal of viruses. 



Environmental factors can alter human defense mechanism that protects against respiratory pathogens.



Such environmental factors can alter one’s defense mechanisms that protects against respiratory pathogens.



Respiratory droplets can be spread 


 indirectly via contact with contaminated surfaces when hands then touch the face. 



Respiratory droplets are large and cannot remain suspended in the air for long periods.



Respiratory droplets are usually dispersed over short distances.



The size of the particles for droplet infections are > 5 μm.



Droplet transmission agents include:  respiratory viruses such as influenza virus, parainfluenza virus, adenoviruses, rhinovirus, respiratory syncytial virus, human metapneumovirus, Bordetella pertussis, pneumococci, streptococcus pyogenes, diphtheria, rubella,


and coronaviruses.



Spread of respiratory droplets can be reduced through wearing of a surgical mask.



Direct contact occurs through skin-to-skin contact, kissing, sexual intercourse, 


contact with soil or vegetation with infectious organisms., and with feces.



Fecal–oral transmission is considered an indirect contact route.



Direct contact transmitted  disease  are called contagious.



Contagious disease is not the same as infectious disease.



All contagious diseases are infectious, but not all infectious diseases are contagious.



Some diseases that are transmissible by direct contact include athlete’s foot, impetigo, syphilis, warts, and conjunctivitis.



Sexually transmitted infection refers to any disease that can be transmitted  during sexual activity with another person, including vaginal or anal sex or through oral sex.



Transmission is either directly between surfaces in contact during intercourse, 


or from secretions.



Direct transmission is the usual route for bacterial infections and those infections causing sores.



Secretion transmission includes semen or the fluid secreted by the excited female which carry infectious agents that get into the partner’s blood stream through tiny tears in the penis, vagina or rectum.



The rectum is a more usual route for viruses.



Anal sex is considerably more hazardous since the penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating.



Sexually transmitted diseases include: 


HIV/AIDS, chlamydia, genital warts, HPV, gonorrhea, hepatitis B, syphilis, herpes, and trichomoniasis.



Sexually transmitted diseases such as HIV and hepatitis B are usually not transmitted through mouth-to-mouth contact, although it is possible to transmit some STDs between the genitals and the mouth, during oral sex. 



HIV can be transmitted via oral sex as can herpes simplex virus 1, usually responsible for oral infections, and in genital infections with type 2 virus.



 Diseases that are transmitted primarily by oral means: direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette: 


includes all of the diseases transmissible by droplet contact, all forms of herpes viruses, Cytomegalovirus infections,


herpes simplex virus (especially HSV-1) and infectious mononucleosis.



Vertically transmitted disease may occur from mother to child, often in utero, during childbirth or during postnatal physical contact between parents and offspring. 



It occurs also via breast milk: HIV, hepatitis B and syphilis.



Other types of infection transmission : medical procedures, such as touching a wound, an injection or transplantation of infected material. 



Iatrogenic transmission includes: Creutzfeldt–Jakob disease by injection of contaminated human growth hormone, and MRSA.



Indirect contact transmission, also known as vehicleborne transmission, involves transmission through contamination of inanimate objects. 



Such  vehicles that may indirectly transmit an infectious agent include: food, water, biologic products such as blood, and fomites such as handkerchiefs, bedding, or surgical scalpels. 



A vehicle may passively carry a pathogen: food or water may carry hepatitis A virus. 



Vehicles may provide an environment in which the agent grows, multiplies, or produces toxin: canned foods providing an environment that supports production of botulinum toxin by Clostridium botulinum.



A vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another.



Vectors are mechanical or biological. 



A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner:  a housefly, lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. 



In this case then pathogen never enters the body of the fly. 



Biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite.



Biological vectors responsible for serious blood-borne diseases: malaria, viral encephalitis, Chagas disease, Lyme disease and African sleeping sickness. 



Biological vectors are usually arthropods, such as mosquitoes, ticks, fleas and lice. 



Vectors are often required in the life cycle of a pathogen, and interrupting the cycle of a pathogen can occur by killing the vector.



In the fecal-oral route, transmission of pathogens in fecal particles pass from one person to the mouth of another person. 



Fecal–oral transmission is considered as an indirect contact route through contaminated food or water. 



Fecal–oral transmission can also operate through direct contact with feces or contaminated body parts, such as through anal sex, and through droplet or airborne transmission through the toilet plume from contaminated toilets.



Fecal–oral disease transmission is mainly due to include lack of adequate sanitation and poor hygiene practices: via foodstuffs or water that has become contaminated, when people do not adequately wash their hands after using the toilet and before preparing food or tending to patients.



Excreta or untreated sewage can pollute drinking water sources, and  people who drink the polluted water can become infected. 



Open defecation also leads to disease transmission via the fecal-oral route: homelessness.



System failures with sewer overflow is a typical mode of transmission for infectious agents such as cholera, hepatitis A, polio, Rotavirus, Salmonella, and parasites.



The tracking of the transmission of infectious diseases, is also termed disease surveillance. 



Surveillance of infectious diseases is responsibility of public health agencies.



Surveillance analysis of aggregate data showing the spread of a disease, relates to  the specialty of epidemiology. 



Human aggregation based on seasonal calendar patterns are related to outbreaks of infectious diseases: school, bootcamp, the annual Hajj etc. 



Cell phones can capture population movements and predict the transmission of certain infectious diseases.



Infectious pathogens must have a way to be transmitted from one host to another to ensure their species’ survival. 



Viruses or bacteria that cause their host to develop coughing and sneezing symptoms have a great survival advantage, as they are much more likely to be ejected from one host and carried to another. 



Similarly, many microorganisms that cause diarrhea have a greater survival by their mechanism of transmission.



Pathogens undergo evolutionary changes in virulence and transmission capabilities over many generationis:


an emerging pathogen may hit its earliest victims especially hard. 



It is usually in the first wave of a new disease that death rates are highest. 



In rapidly fatal diseases, the host may die before the microbe can be passed along to another host. 



Vertical transmission refers to acquisition of infection from parents, usually mothers.



Vertical transmission can be intracellular, such as transovarial, or extracellular through post-embryonic contact between parents and offspring.



There is a link between temperature and relative humidity and viral transmission, with greater persistence and infectIvity of influenza A virus and cold temperatures and low relative humidity.



The environment may reduce  mucociliary clearance by desiccation of upper airways, increased ability of viruses with a lipid envelope and dry air and impact of relative humidity on droplet dynamics.



A cold temperature and low relative humidity also increase the transmission of other respiratory viruses, including respiratory syncytial virus, human rhinovirus, and avian influenza virus.



Influenzae has distinct seasonality in winter months in temperate climate, but epidemics are less seasonal at locations closer to the equator.



Epidemics of RSV are seasonal, starting in late summer in the tropics, and increase prominence during the winter in temperate climates.



Parainfluenza virus epidemics occur primarily in spring and early summer.



Meta-Pneumovirus epidemics occur in late winter and spring in most temporal areas.



Lower temperatures are associated with higher disease activity of influenza virus and RSV.



Covid-19 most often spreads via droplets when infected persons exhale, cough, sneeze, or talk. 



A less prominent mechanism of transmission, airborne transmission of Covid-19 is also suspected.



The Covid-19 virus may remain infectious in aerosols for hours and on surfaces up to days.



Transmission of Covid-19 studies indicated that higher temperatures increase growth rate, and relative humidity was associated with a non-significant decrease in growth rate, and ultraviolet light has the strongest effect in decreasing its growth rate.









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