# what makes something more or less contagious? **URL:** \ **Category:** Factual Questions\ **Created:** [May 8, 2020, 1:23pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406 "2020-05-08T13:23:23Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [May 8, 2020, 1:23pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406/1 "2020-05-08T13:23:23Z")
What factor(s) determine the degree to which a disease is contagious? Does it have to do with the actual number of particles transferred? I.e. do some diseases require a more massive dose of infectious agents in order to trigger the disease? Do some diseases only require very few bacteria or viruses in a host in order to bring about disease? Is that why the length of time of contact seems to be a factor in transmitting disease? Do some infectious agents mover more efficiently from one person to another? I believe that conjunctivitis is considered a very contagious disease. What makes it so? And what do we know about the covid virus? I know we have some docs on the SDMB and I suppose they’re too busy these days to run a course here on contagious diseases, but this is all too mysterious for something that’s part of our every day lives. ---
**Author:** ![zimaane](https://avatars.discourse-cdn.com/v4/letter/z/a3d4f5/32.png) [@zimaane](https://boards.straightdope.com/u/zimaane)\ **Post date:** [May 8, 2020, 1:43pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406/2 "2020-05-08T13:43:34Z")
Not a physician, just an interested layman It’s a complicated question but two important factors are how well the infectious agent survives outside the body and how the disease effects behavior of the sufferer First, there is a lot of variation in how well an agent like a virus can remain viable outside a host. HIV, the virus that causes AIDS, does not last long outside a human host. That’s why it can’t be transmitted by casual contact. Measles, on the other hand, can linger in the air and remain intact. You can catch it by being in the same room that someone with measles had been in hours prior. Second, there is the impact on behavior. Some illnesses, such as flu, will make someone feel very sick soon after being infected. The victim will stop moving around and not contact many people, limiting the spread. The scary thing about COVID 19 is that it seems to last quite a while, especially on surfaces, AND it does not make its victim sick right away, giving them plenty of time to spread the virus around. ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [May 8, 2020, 2:17pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406/3 "2020-05-08T14:17:25Z")
Good. Thanks. Two factors I had completely overlooked. I’ll keep watching this space. ---
**Author:** ![Wesley\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wesley_clark/32/20581_2.png) [@Wesley\_Clark](https://boards.straightdope.com/u/Wesley_Clark)\ **Post date:** [May 8, 2020, 2:46pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406/4 "2020-05-08T14:46:36Z")
The number of particles needed to cause an infection varies by infectious pathogen. I believe norovirus only requires 10 viral particles while anthrax requires 10,000 to cause an infection. Also one factor is what % of patients are symptomatic. With SARS, one reason it was so much easier to control is that people aren’t contagious until they develop symptoms. Due to that it is much easier for people to quarantine or be diagnosed. With COVID-19 a lot of people are contagious either without symptoms or before symptoms, which makes spreading the disease easier. Its the same with the flu, most flu cases are asymptomatic. Which means people can spread it w/o knowing it. > **[NEJM Journal Watch: Summaries of and commentary on original medical and...](https://www.jwatch.org/fw108600/2014/03/17/most-flu-asymptomatic)** > > NEJM Journal Watch reviews over 250 scientific and medical journals to present important clinical research findings and insightful commentary There are also factors like how close together people are. A cruise ship, subway car or military barracks will have more contagiousness than an outdoor rural area for respiratory diseases. I’m sure people way way smarter than me have figured out ‘all’ the factors in contagiousness, but I’m not personally sure what they all are. - how many particles needed to cause infection - Are people infectious w/o symptoms or pre-symptomatic - Are people congregated in enclosed areas (supposedly this is what made the 2nd wave of the spanish flu so deadly) - how long does the infection last on surfaces or in the air - how is the disease transmitted (surfaces, air, sex, etc) - how severe is the disease (less severe diseases spread easier since people can still walk around. However diseases spread by vectors like mosquitoes or fleas can be both infectious and deadly) ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [May 8, 2020, 2:50pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406/5 "2020-05-08T14:50:36Z")
Possibly of interest: [What We Do and Do Not Know About COVID-19’s Infectious Dose and Viral Load](https://www.discovermagazine.com/health/what-we-do-and-do-not-know-about-covid-19s-infectious-dose-and-viral-load) > [@](#): > > Scientists do not know how many virus particles of SARS-CoV-2 are needed to trigger infection. COVID-19 is clearly very contagious, but this may be because few particles are needed for infection (the infectious dose is low), or because infected people release a lot of virus in their environment. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [May 8, 2020, 3:03pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406/6 "2020-05-08T15:03:04Z")
Indoor vs. outdoor - which is why some people (?) were speculating about intense internal doses of UV… Apparently UV light, as in sunlight, does a number on covid19 pretty fast. Within in minutes the virus is dead if floating in the air outdoors in sunlight. So, indoors transmission from cough is far more likely. Note how several members of a church choir were infected in one place - it’s likely the vocal effort of singing produces lots of minute droplets containing the virus. Also remember the bubonic plague in the 1340’s was spread by cough droplets, but also by rats and flea bites, something a bit more prevalent with 1300’s-level hygiene. Plenty of other diseases like malaria are also spread by insect bites - a mosquito will inject a small amount of possibly infected saliva to numb the bite area so it can linger long enough to suck up a stomach-full. Tsetse flies in central Africa are notorious for spreading sleeping sickness with their bites. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [May 8, 2020, 3:12pm UTC](https://boards.straightdope.com/t/what-makes-something-more-or-less-contagious/853406/7 "2020-05-08T15:12:12Z")
And there’s this: > **[Coronavirus Found In Men's Semen - Slashdot](https://science.slashdot.org/story/20/05/08/0549227/coronavirus-found-in-mens-semen)** > > An anonymous reader quotes a report from CNN: The new coronavirus can persist in men's semen even after they have begun to recover, a finding that raises the possibility the virus could be sexually transmitted, Chinese researchers said Thursday. A...