condition that increases the risk of severe covid-19, and people who live with someone from one of these groups from engaging in kissing, casual sex, or other activities, and has recommended wearing face mask during sex. the risk of transmission from all size droplets and aerosols is lower in indoor spaces with good ventilation. the risk of outdoor transmission is low. transmission events occur in workplaces, schools, conferences, sporting venues, dormitories, prisons, shopping facilities, and ships, as well as restaurants, passenger vehicles, religious buildings and choir practices, and hospitals and other healthcare settings. a superspreading event in a skagit county, washington, choral practice resulted in 32 to 52 of the 61 attendees infected. an existing model of airborne transmission (the wells-riley model) was adapted to help understand why crowded and poorly ventilated spaces promote transmission, with findings supported by aerodynamic analysis of droplet transfer in air-conditioned hospital rooms. airborne transmission also occurs in healthcare settings; long-distance dispersal of virus particles has been detected in ventilation systems of a hospital. some scientists criticized public health authorities, including the who, in 2020 for being too slow to recognize airborne (aerosol) transmission of covid-19 and to update their public health guidance accordingly. by mid-2020, some public health authorities had updated their guidance to reflect the importance of airborne transmission. the who updated it only by 23 december 2021. medical procedures designated as aerosol-generating procedures there is concern that some medical procedures that affect the mouth and lungs can also generate aerosols, and that this may increase the infection risk. some medical procedures have been designated as aerosol-generating procedures (agps), but this been done without measuring the aerosols these procedures produce. the aerosols generated by some agps have been measured and found to be less than the aerosols produced by breathing. less virus (strictly speaking, viral rna) has been found in the air near intensive care unit (icus) with covid-19 patients than near rooms with covid-19 patients that are not icus. patients in icus are more likely to be subject to mechanically ventilation, an agp. this suggests that in hospitals, areas near icus may actually pose less risk of infection via aerosols. this has led to calls to reconsider agps. the who recommends the use of filtering facepiece respirators such as n95 or ffp2 masks in settings where aerosol-generating procedures are performed, while the u.s. cdc and the european centre for disease prevention and control recommend these controls in all situations related to covid-19 patient treatment (other than during crisis shortages). there is a research that suggests that variation in airway resistance, as measured by cfd (computational fluid dynamics), may be a useful tool for predicting the forecast of critically ill covid-19 patients. rarer modes of transmission surface (fomite) transmission surfaces that are often touched such as door handles may transmit covid-19, although is not thought to be the main way the virus spreads. a person can get covid-19 by touching a surface or object that has the virus on it (called a fomite), and then touching their own mouth, nose, or eyes, but it is not the main mode of transmission, and the risk of surface transmission is low. as of july 2020, "no specific reports which have directly demonstrated fomite transmission" although "people who come into contact with potentially infectious surfaces often also have close contact with the infectious person, making the distinction between respiratory droplet and fomite transmission difficult to discern." each contact with a surface contaminated with sars-cov-2 has less than a 1 in 10,000 chance of causing an infection. various surface survival studies have found no detectable viable virus on porous surfaces within minutes to hours, but have found viable virus persisting on non-porous surfaces for days to weeks. however, surface-survival studies do not reflect real-world conditions, which are less favorable to the virus. ventilation and changes in environmental conditions can kill or degrade the virus. for example, temperature, humidity, and ultraviolet radiation (sunlight) all influence reductions in viral viability and infectiousness on surfaces. fomite transmission risk is also reduced because the virus does not transfer efficiently from the surface to the hands, and then from the hands to the mucous membranes (mouth, nose, and eye). the initial amount of virus on the surface (i.e., the viral load in respiratory droplets) also affects fomite transmission risk. hand washing and periodic surface cleaning impede indirect contact transmission through fomites. fomite transmission can be easily prevented with use of regular household cleaners or disinfection. when surface survival data and factors affecting real-world transmission are considered, "the risk of fomite transmission after a person with covid-19 has been in an indoor space is minor after 3 days (72 hours), regardless of when it was last cleaned." animal vectors although the covid-19 virus likely originated in bats, the pandemic is sustained through human-to-human spread, and the risk of animal-to-human spread of covid-19 is low. covid-19 infections in non-human animals have included companion animals (e.g., domestic cats, dogs, and ferrets); zoo and animal sanctuary residents (e.g., big cats, otters, and non-human primates); mink in mink farms in multiple countries; and wild white-tailed deer in numerous u.s. states. most animal infections came after the animals were in close contact with a human with covid-19, such as an owner or caretaker. experimental research in laboratory settings also shows that other types of mammals (e.g., voles, rabbits, hamsters, pigs, macaques, baboons) can become infected. by contrast, chickens and ducks do not seem to become infected with, or spread, the virus. there is no evidence that the covid-19 virus can spread to humans from the skin, fur, or hair of pets. the u.s. cdc recommended that pet owners limit their pet's interactions with unvaccinated people outside their household; advises pet owners not to put face coverings on pets, as it could harm them; and states that pets should not be disinfected with cleaning products not approved for animal use. if a pet becomes sick with covid-19, the cdc recommends that owners "follow similar recommended precautions as for people caring for an infected person at home." people sick with covid-19 should avoid contact with pets and other animals, in the same manner that people sick with covid-19 should avoid contact with people. vectors for which there is no evidence of covid-19 transmission mother to child the is no evidence for intrauterine transmission of covid-19 from pregnant women to their fetuses. studies have not found any viable virus in breast milk. breast milk is unlikely to spread the covid-19 virus to babies. noting the benefits of breastfeeding, the who recommends that mothers with suspected or confirmed covid-19 should be encouraged to initiate or continue to breastfeed, while taking proper infection prevention and control measures. food and water no evidence suggests that handling food or consuming food is associated with transmission of covid-19. the covid-19 virus had poor survivability on surfaces; less than 1 in 10,000 contacts with contaminated surfaces, including non-food-related surfaces, lead to infection. as a result, the risk of spread from food products or packaging is very low. public health authorities recommend that people follow practice good hygiene by washing hands with soap and water before preparing and consuming food. the covid-19 virus has not been detected in drinking water. conventional water treatment (filtration and disinfection) inactivates or removes the virus. covid-19 virus rna is found in untreated wastewater, but there is no evidence of covid-19 transmission through exposure to untreated wastewater or sewerage systems. there is also no evidence that covid-19 transmission to humans occurs through water in swimming pools, hot tubs, or spas. other while sars-cov-2 rna has been detected in the urine and feces of some persons infected with covid-19, there is no evidence of covid-19 transmission through feces or urine. covid-19 is not an insect-borne disease; there is also no evidence that mosquito are a vector for covid-19. covid-19 is not a sexually transmitted infection; while the virus has been found in the semen of people who have covid-19, there is no evidence that the virus spreads through semen or vaginal fluid, however transmission during sexual activities is still possible due to proximity during intimate activities which enable transmission through other paths. transmission rate, patterns, clusters many people do not transmit the virus, but some transmit to many people, and the virus is considered to be "over dispersed" – the transmission rate has high heterogeneity. "super-spreading events" occur from this minority of infected people, generally indoors and usually in high-risk venues where people remain in close proximity and poor ventilation for an extended period, such as restaurants, nightclubs, and places of worship. such crowded conditions enable the virus to spread easily via aerosols, they can create clusters of cases, where infections can be traced back to an index case or geographical location. another important site for transmission is between members of the same household, as well as hospitals due to the abundance of pathogens present. traffic vehicles are also a site for transmission, since the control of the pathogen there is harder due to the weak ventilation system and the high density of people. emergency departments are also great sites for transmission of covid-19. the dispersion of respiratory droplets can be influenced by various factors, including the ventilation system, the number of infected patients, and their movements, which highlights the importance of proper ventilation and air filtration systems in reducing the spread of covid-19 within an emergency department setting. covid-19 is more infectious than influenza, but less so than measles. estimates of the number of people infected by one person with covid-19—the basic reproduction number (r0)—have varied. in november 2020, a systematic review estimated r0 of the original wuhan strain to be approximately 2.87 (95% ci, 2.39–3.44). the r0 of the delta variant, which became the dominant variant of covid-19 in 2021, is substantially higher. among five studies catalogued in october 2021, delta's mean estimate r0 was 5.08. an infected individual spreading covid-19 to healthy individuals the flow generated from a cough persists for 0.8 seconds. the droplets are ejected over a rapider period of time than the flow-field produced by the cough. the regression in the average droplet velocity in sneezes is steadier than in coughs and is comparable to the reduction in volumetric flow rate. for the first 0.1 seconds the cough droplets follow the jet flow ejected with a trajectory away from the individual. therefore, the airflow drives only the initial phase of droplet motion. after that, for the bulk of the cough, most droplets display aerosolised behaviour, therefore a cough should not be estimated by as an unvarying jet. the sneeze droplets have greater momentum, with a mean droplet velocity of 2 m/s to 5.4 m/s across different individuals, which drives the droplets to trail the airflow stream direction for most of the sneeze interval and appear to display aerosolised behaviour at a much future stage. temperature is also a factor that affects the transmissibility of the virus. at elevated temperatures and low virus concentration rates the virus is in its weak state and the spreading of it is strenuously. at low temperatures and excessive virus concentration rates the virus is in its robust state and the spreading of it is unchallenged. effect of face masks and face shields face masks are the principal mode of protection of covid-19 after the vaccine. there are many types of face masks: surgical mask, two-layered face mask, face shield, and n-95 face mask. the surgical mask is the least effective means of preventing particle leakage, since the leaked particles from the sneeze travel a distance of 2.5 ft. the combination of a surgical mask with a face shield restricts the particles’ forward motion notably. the two-layered face mask has noticeable leakage in the forward direction, but with the addition of a cotton stitched of the mask there is significantly less leakage of particles. the combination of a two-layered face mask and a face shield effectively restricts the leakage in the forward direction. the face shield enables particles to escape from below it, and thus it is not recommended for protecting the spreading of the virus. the n-95 face mask restricts completely the forward leakage of particles, but there is a significant amount of particles escaped from the gap between the nose and the mask. n95 face mask none of the protective