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Infectious respiratory diseases spread when a healthy person comes in contact with virus particles expelled by someone who is sick — usually through a cough or sneeze. The amount of particles a person is exposed to can affect how likely they are to become infected and, once infected, how severe the symptoms become.

The amount of virus necessary to make a person sick is called the infectious dose. Viruses with low infectious doses are especially contagious in populations without significant immunity.


The minimum infectious dose of SARS-CoV-2, the virus that causes Covid-19, is unknown so far, but researchers suspect it is low. “The virus is spread through very, very casual interpersonal contact,” W. David Hardy, a professor of infectious disease at Johns Hopkins University School of Medicine, told STAT.

Alex Hogan/STAT

A high infectious dose may lead to a higher viral load, which can impact the severity of Covid-19 symptoms.

Viral load is a measure of virus particles. It is the amount of virus present once a person has been infected and the virus has had time to replicate in their cells. With most viruses, higher viral loads are associated with worse outcomes.


“The more viral particles that get into the lungs, the more damage to the lungs that is probably happening,” said Hardy.

One study of Covid-19 patients in China found that those with more severe symptoms tended to have higher viral loads.

“It’s not proven, but it would make sense that higher inoculating doses will lead to higher viral loads, and higher viral loads would translate into more pathogenic clinical courses,” said Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center.

Alex Hogan/STAT

People with higher viral loads may also shed more whole viruses, which makes them more contagious, compounding the danger of spreading disease more widely.

If exposure to higher doses, or even frequent low doses, of SARS-CoV-2 does lead to worse health outcomes, there are significant implications for health care workers who are routinely exposed to Covid-19 patients.

“Someone caring for large numbers of patients on the wards, if they’re not wearing PPE [personal protective equipment], there might be a high frequency of exposure as well as a high dose of exposure,” Barouch said.

In Italy, a country particularly hard-hit by the virus, about 9% of reported cases were health care workers. Here in the U.S., 10% of Covid-19 cases in California were health care workers, according to the California Department of Public Health.

  • This is purely based on my own assumptions. is there any testing being done on building immunity to the virus by giving minute or extremely small doses and waiting to build immunity for that tiny dose. then incrementally increasing the dose until full immunity is built. just like using medication that you incrementally increase dosage until the body adjusts to it.

    • Nash – the way your body reacts to a virus would be totally different than the way it reacts to medication, and the virus inside your body will replicate, so that is not likely to work.

    • But this concept is the same as the “herd immunity” except you expose the person to a much less amount. in other words you control the exposure. I call it “controlled herd immunity”

    • You can be infected with ANY amount of virus – so, even a tiny amount might infect you, not give you immunity.
      Another known immune effect can be developing a tolerance, rather than immunity, for things you get small amount of over a long period of time – the theory is, if something in your environment permanently provoked to much immune response you would just be having allergic reaction all the time.

  • If we can get the disease from a cough or sneeze (so they said) then the most basic logic is to have some kind of wall between your nose and mouth and said particles/droplets. I knew there was no sense to what they were saying and figured they were doing so due to a fear that masks would all be bought up from panic and hoarding. Now their reason that we should wear masks is so that we will remember not to touch our faces. A bit ridiculous.
    Anyone who has ever worked on heating and/or air conditioning in a large building knows that they do create air movement. That’s how they work. Also, where I live, gusty winds are a very common occurrence. When I go out for a walk, if someone is smoking a cigarette a half to even one block upwind, it’s not unusual for me to be able to smell it. First it was three feet, then it was six feet and now I hear thirteen. You can’t believe everything you’re told. Sometimes you need think for yourself. Particularly for something this new, this deadly, where there is so much more to learn.

  • I don’t want to sound foolish but is it possible to raise a patient’s body temperature to around 104-105 to help fight off the Virus? Only looking at the Host the Horseshoe bat that releases molocules though flight Increased heart beat and Elivated body temperature to fend off viral effect?

  • Please do not accuse me of spreading misinformation. Your post follows the guidelines of the CDC of a few weeks back – before they were COMPLETELY REVERSED to tell people to wear face masks.
    If you look at the foreign media, you can see many very prominent virologists endorsing airborne transmission.
    And your claim that 6 feet is “how far water droplets can travel before they hit the floor” is highly implausible on it’s face- do you mean in completely still air, where the only motion in the air is Brownian motion, or would a ten mile per hour wind, with the infected person standing upwind of you, carry the droplets a little further? How about a 20 mile per hour wind, which is a good breeze but hardly rare? Because the implication, in giving out this advice as you have, is that under any normal conditions, 6 feet is enough – but that is flat out ridiculous on it’s face – water droplets are clearly spread by wind, much further than 6 feet.
    YOU are spreading misinformation, and it is VERY DANGEROUS misinformation.

    • Dave Wyman -Who goes out in a 20 mph wind? People who have to go to work when the wind is blowing 20 mph.

      But since you pointed out the wind will disperse the virus – ANY amount of virus can infect you, according to some research. I know you will hear the claim you need a certain amount of virus to get infected, but there is no good theoretical basis for that, and some testing of other diseases indicates it is not likely to be true.
      The thing is, not so much what is happening outside – I make a point of staying upwind of everyone outside- but as soon as you go inside, you are breathing in the air everyone else breathed out, and it will contain virus.

  • It is misinformation to say that the virus particles hang around in the air at large stores and you can pick them up from someone walking through an aisle some time before you. The reason experts give a six-foot safe distance is because that is how far water droplets can travel before they hit the floor. The micro particles do not hang around in the air in one place. They are blown away by any kind of airflow, and large stores are well ventilated.

    • I am not trying to get in a war with you, but please see my reply above, you are actually spreading misinformation, I am not, and your misinformation is very dangerous to anyone who believes it, and therefore does not protect themselves. I researched my statements carefully before making them. The church choir practice, the models developed, the opinions of prominent researchers, all of them went into my statements. You seem to have blindly followed the CDC advice, which, this is sad to say, but was deliberate misinformation which was harmful to the public health. They knew the disease could be better limited if everyone wore face masks, but told us it was not necessary. And now they claim it is desirable. Why? As best I can guess, they knew there were not enough face masks for everyone and were worried about hoarding so that doctors would not have any.

    • And, just to use some basic logic. So, the water droplets will not travel more than 6 feet ? So, what if I am traveling INTO the cloud of them left by the infected person? How long do they take to fall to the ground?
      And, how about that ventilation? Can it blow the water droplets TO me? Or keep them suspended in the air longer?
      Without the right answers to these questions your claims do not hold up, but, even WITH the right answers, which do not make sense – even WITH all those answers on your side, they are many prominent virologists who say it is transmitted by air alone.

  • So does this mean that if you pick up a small bit of degraded virus on a surface somewhere and rub it in your eye or get it in your mouth, you will likely get a little sick, but if someone with the virus sneezes in your face, shooting many ” healthy” virus molecules into your system, you will get sicker, and if someone sneezes in your face everyday, you will get dangerously sick?

    If so can we be reassured that we will not become seriously ill as long as we wear masks, wash our hands and keep a decent distance from each other?

    • I am not a scientist or doctor, but I read up a lot on this and I say the answer is NO, you may get it anyway, but your chances are improved – but remember, IT IS AIRBORNE, EASILY AIRBORNE, NO ONE NEEDS TO BE COUGHING AT ALL – so, a “decent distance” is hundreds of feet away if the infected person is upwind of you, or walked through the supermarket aisles before you. Or Costco aisles, or drug store aisles – the air in those places is contaminated for hours after an infected person goes through, they do not need to cough or sneeze and they often have no signs of infection.

  • I am not a biologist of any type, but I have read peer reviewed papers which state there is NO “minimum infectious dose”, that concept is simply wrong.
    Those researchers could have been wrong, and it could certainly be a rarity that a person gets infected by one virus particle, but those researchers believed it was at least possible.
    Now, about the amount of virus you originally take in to get infected relating to how sick you eventually get – can any scientist or MD weigh in on this? Since a virus can reproduce rapidly, this seems unlikely on it’s face, but particularly with the most serious cases of this virus, the patients are often seemingly OK -sick but not dangerous sick – for weeks, then rather abruptly worsen and get extremely sick.
    One study I saw indicated the viral load in those very sick people was closely related to how sick they were, but the widespread theory is, that is due to cytokine storm. So, my theory is, the severe illness some patients have, two weeks after they got infected, or more even, has very little to do with how much virus they originally took in. Taking in a very small amount would in theory give the immune system time to “learn” how to fight the virus – to create antibodies to fight it – but the antibodies, or some of them anyway, seem to be part of the problem creating the cytokine storm. And in any case, even if that theory is wrong, after the virus had over two weeks to multiply, it seems very unlikely the amount originally taken in has much relevance to what is happening in the patient’s body two weeks later, even if the amount taken in during research on totally different virus species was relevant to the disease progression in those species. Covid19, life threatening cases at least, seem to follow a different course.

    • I wanted to put out here another thing – from what I read, the caution about a vaccine, such that they talk about one not being available for 18 months, seems to be due to ADE – Antibody Dependent Enhancement which was identified as one of the dangers with an experimental SARS vaccine.
      ADE is where the person’s antibodies to the virus, which should lock up virus particles and thereby fight the diseases, are used by the virus to gain access to cells they would not otherwise have access to – so, the antibodies (some of them anyway, remember the immune system may make more than one antibody in response to an infection) attach to cells which the virus can now infect by attaching to the antibody first.

      So, that is a known effect with Dengue fever, which is not very lethal on first infection, but if you’ve had it before becomes very dangerous the second time you get infected.

      So, they are scared of this with any SARS 2 vaccine – (I am sorry, but calling it Covid 19, to me, let’s the Chinese government off the hook too much).

      The thing is however, obviously, people recover from SARS2, and if the only effect was ADE, how could that even happen? That I have not been able to learn yet. I would think the body is making some antibodies which do NOT create an effect of ADE, and the vaccine work is to identify them and put them into production – from what I understand also, it is possible for aritificial antibodies to block SARS2, not to enhance it – an artificial antibody would attack to the part of the virus which attaches to the cell, but NOT attach it’s other end to the cell – so that virus particle is now locked up. I am not sure about convalescent plasma – if it could cause ADE or not – for that matter, reading up on it, it was not clear that SARS2 immunity can cause ADE – it was identified as a problem with SARS, but not SARS 2 as far as I can find.

    • Steve, hot air from belief -oriented ( as opposed to evidence -oriented) people can be toxic to everyone. Some believe that the sky is Carolina Blue because God is a Tarheel. (Evidence: He is !)

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