ID50, or median infectious dose, is the number of pathogen particles needed to infect 50% of a population exposed under specific conditions. It’s one of the core measurements in microbiology for quantifying how contagious a germ actually is. A lower ID50 means a pathogen is more infectious: it takes fewer organisms to make people sick. The values range enormously, from as few as 1 to 10 particles for norovirus to over a million for cholera.
What ID50 Actually Measures
ID50 captures a simple but important idea: if you expose a group of people (or lab animals) to increasing amounts of a pathogen, at what dose does half the group become infected? That threshold is the ID50. It doesn’t tell you how severe the resulting illness will be or whether anyone dies. It strictly measures the ability of a pathogen to establish an infection, to get past the body’s barriers, survive, and start multiplying.
This makes it different from LD50, which is the dose that kills 50% of an exposed group. A pathogen can have a very low ID50 (highly infectious) but a high LD50 (rarely lethal), or vice versa. Norovirus, for example, infects easily but almost never kills healthy adults. Anthrax spores are harder to contract in some forms but far more dangerous once infection takes hold.
Why ID50 Varies So Widely Between Pathogens
The range of ID50 values across common human pathogens is striking:
- Norovirus: 1 to 10 particles
- Shigella dysenteriae: around 10 organisms
- Salmonella Typhi: fewer than 1,000
- SARS-CoV-2: estimated at roughly 100 particles
- Vibrio cholerae (cholera): 1 million to 100 billion organisms
Norovirus is extraordinarily infectious because just a handful of viral particles can cause a full-blown stomach illness. Cholera sits at the opposite extreme. That’s largely because cholera bacteria are highly vulnerable to stomach acid. The vast majority of ingested bacteria die before reaching the intestines, so it takes an enormous dose for enough to survive and cause disease.
Stomach acid is one of the biggest factors driving these differences. Pathogens that resist acid, like Shigella, E. coli, and the cyst forms of parasites such as Cryptosporidium, tend to have very low ID50 values. They pass through the stomach largely intact, so even a tiny dose can establish infection.
Factors That Shift the Number
ID50 isn’t a fixed property of a pathogen. It changes depending on the route of exposure, the host’s immune status, and environmental conditions. The same organism can have a dramatically different ID50 when inhaled versus swallowed. Anthrax illustrates this clearly: cutaneous anthrax (through a skin break), gastrointestinal anthrax (from swallowing spores), and pulmonary anthrax (from inhaling spores) each represent a distinct infection pathway with different dose requirements and very different outcomes.
Host factors matter just as much. People who are immunocompromised, whether from HIV, chemotherapy, or other conditions, can become infected at doses well below the standard ID50. The same applies to infants whose immune systems haven’t fully developed and to older adults experiencing age-related immune decline. Prior vaccination or a previous infection can raise the effective ID50 substantially, because the immune system recognizes and neutralizes the pathogen before it can establish itself.
Even something as simple as taking antacid medication can lower the effective ID50 for foodborne pathogens. By reducing stomach acid, the body’s first chemical barrier becomes less effective, and organisms that would normally be killed in the stomach survive to reach the intestines.
How ID50 Is Calculated
Researchers determine ID50 experimentally by exposing groups of animals (or, rarely, human volunteers) to a series of increasing doses and recording what fraction of each group becomes infected. The two most commonly used statistical methods are the Reed-Muench method and the Spearman-Kärber method.
Both work with serial dilutions, a process where the pathogen is progressively diluted so each group receives a known, decreasing concentration. The Reed-Muench method finds the 50% threshold by interpolating between the highest dose where more than half the group was infected and the lowest dose where fewer than half were. The Spearman-Kärber method takes a slightly different mathematical approach, averaging across all dilution levels and weighting by the proportion of infected animals at each step. Both yield essentially the same result for well-designed experiments.
The dose itself can be measured in different units depending on the pathogen: colony-forming units for bacteria, plaque-forming units for viruses, individual oocysts for parasites, or gene copies detected by molecular methods. This is worth keeping in mind when comparing ID50 values across studies, because different counting methods don’t always line up perfectly.
How Public Health Agencies Use ID50
ID50 is a cornerstone of microbial risk assessment. Agencies like the USDA and EPA use it to evaluate how dangerous specific pathogens are in food and water supplies. When regulators set acceptable contamination limits for drinking water or ready-to-eat food, the ID50 of the relevant pathogens is one of the key inputs in their models.
In risk assessment, there’s no single “safe” threshold below which infection is impossible. Instead, regulators work with probability distributions: at any given dose, there’s some probability of infection, and that probability varies from person to person. The ID50 serves as a benchmark, a reference point around which those probability curves are built. A pathogen with an ID50 of 10 particles demands far stricter contamination controls than one with an ID50 of a million.
During disease outbreaks, estimated ID50 values also help epidemiologists model transmission. For SARS-CoV-2, computational analysis of how the virus spreads through respiratory droplets led to an estimated ID50 of roughly 100 particles. That low number helped explain why the virus spread so efficiently in indoor settings, even with brief exposure times, and informed ventilation and distancing guidelines.
ID50 as a Measure of Infectivity, Not Severity
One common point of confusion is treating ID50 as a measure of how dangerous a pathogen is overall. It only captures one dimension: how easily the pathogen establishes infection. It says nothing about whether that infection will cause mild symptoms, severe illness, or death. A pathogen with an extremely low ID50, like norovirus, causes miserable but self-limiting gastroenteritis in most people. A pathogen with a higher ID50, like certain strains of E. coli, can cause life-threatening kidney failure.
Infectivity (captured by ID50), pathogenicity (the ability to cause disease once infection occurs), and virulence (the severity of that disease) are three separate characteristics. A complete picture of how threatening a pathogen is requires all three. ID50 gives you the first piece: how much of the organism it takes to get a foothold in the body.