What are the transmission routes of respiratory diseases

The currently clear transmission routes for respiratory diseases mainly include respiratory droplet transmission, aerosol transmission, and contact transmission. Some special pathogens also have non-mainstream transmission routes through digestive tract and mucosal contact that await further verification.

I have been working on the prevention and control of infectious diseases at the grassroots level for almost 8 years. I have dealt with at least thirty or forty cases of respiratory disease outbreaks, and most of them cannot avoid these methods. Last winter, there was an outbreak of influenza A in the primary school in the district. The two children who were first diagnosed coughed without covering their mouths during class. Three children who were separated by less than one meter in the front and back rows developed fever symptoms that day. This is the most typical droplet transmission - large particles of secretions are sprayed out by patients when they cough, sneeze, or speak loudly. They are like small droplets of water thrown casually at people. They usually float up to 1-2 meters before falling to the ground. People in close contact will be infected if they inhale it directly. This is also the most common daily transmission scenario.

Many people easily confuse aerosol transmission with droplet transmission. In fact, the main difference between the two is particle size and suspension time. We previously dealt with an epidemic in a chess and card room. Among the dozen confirmed cases, an aunt just went in to get a key from the boss. She stayed there for less than 5 minutes and did not talk to the sick patients at close range. The final investigation determined that it was aerosol transmission. The carriers of this kind of transmission are tiny secretion particles with a diameter of only a few microns, which can be suspended in a closed airless space for ten minutes or even longer. It is a bit like spraying mosquito repellent in a closed room, and you can still smell it after half an hour. The virus is attached to these small particles, and people may be infected after inhaling it. However, there are still some differences in the academic community about the transmission scenarios of aerosols. Some scholars believe that only long-term exposure to confined spaces poses a risk of infection. Some epidemic investigation teams have also found evidence of aerosol transmission in semi-open places with extremely poor ventilation (such as crowded open-air queuing spots and street snack bars with no ventilation). There is currently no completely unified conclusion. As long as you pay attention to opening windows for ventilation and wearing masks in crowded places, you can basically protect yourself.

Another thing that is easily overlooked is contact transmission. Last month, I met a young man who had the flu. He said that he wore an N95 all the time when he went out. He couldn't figure out how he could be infected. After careful questioning, I found out that he touched the buttons of the company elevator when he was at work, rubbed his nose several times without washing his hands, and even grabbed rice balls with his hands at noon. In fact, after a patient's droplets fall on surfaces such as elevator buttons, door handles, and desktops, many respiratory viruses can survive for several hours to several days. After touching the hands with the virus, and then touching the mucous membranes such as the mouth, nose, and eyes, it is equivalent to actively "passing" the virus into one's own body. This kind of transmission is mostly linked to poor hand hygiene habits.

As for the paths of sewer aerosol transmission and fecal-oral transmission that have been mentioned many times during the previous COVID-19 epidemic, there is currently no unified consensus in the academic community. There are many cases of infection without contact when going up or downstairs. Some scholars believe that the virus forms aerosols along the sewer pipes and enters the household. Some experts believe that such cases often also have other unexamined contact exposure possibilities, and more research evidence is needed to support them. In fact, you don’t need to worry too much about the specific transmission route in daily life. If you do these three things: wearing a mask, washing hands frequently, and opening windows more often, you can block more than 90% of the risk of respiratory infections.