# Why minimum infective dose of a virus is not just 1 particle?

I've stumbled upon a term "minimum infective dose" (for example, of HIV), but don't exactly understand what is it? Why it is not always equal to 1 virus particle? I expect 1 lucky virus to be able to start infection just like 1 lucky spark can start a fire.

The following options comes to mind:

• Multiple particles of the same virus need to act in cooperation to trigger their reproduction;
• There is some 100% working, but limited use immunity that knocks down first N viruses;
• Infection probability lower than some theshold % is not considered by statistics.

In general, how infection probability and severity correlates with the number of particles? Is it linear (double the number of viruses => double probability or infection) or something trickier?

• You just answered your own question. "I expect 1 lucky virus to be able to start infection just like 1 lucky spark can start a fire." How often does a spark - a single spark - start a fire? How many times will the flint be struck to get enough sparks that one will land on the exact spot needed to start a fire? The rest of your answer depends on the virus and the host. – anongoodnurse Sep 1 '17 at 0:51
• Then it should be said not "minimum infective dose", but a "minimum dose that cause infection with N% certainity" or "probability of infection from a single virus". – Vi. Sep 1 '17 at 1:18
• The meaning is implied in the term. Remember that scientific inference is based on repeated observations. Observations closer to the mean are more likely to be given more importance than rare outliers. – Roni Saiba Sep 1 '17 at 9:26

From Merriam Webster:

the smallest quantity of infective material that regularly produces infection

(emphasis mine)

The word "minimum" here is not meant as a strict minimum it is meant as a statistical bound, the absolute probability for which depends on the types of tests done and their inherent accuracy/inaccuracy.

Reasons that this "minimum" is greater than 1 could include:

1. Probability that a given particle actually finds a suitable host cell, enters the cell, and replicates; failures can occur at many stages along that chain

2. Rate of immune system reaction/response. At low exposure levels, the immune system may detect and clear the infection at a rate faster than it can spread, such that the infection stays asymptomatic. Some vaccines use weakened or low doses of pathogens specifically to evoke an immune response. The intent is that the weakened pathogen will not spread fast enough to overcome the immune response and will not produce a full disease.

3. Clearance of the infectious agent before it reaches symptomatic levels. This probably applies more to bacteria but I don't see why it couldn't also apply to viruses. I think the best example is exposure to bacteria that cause gastrointestinal infections like E.coli or Salmonella. Small numbers may grow and multiply in the gut, but they grow slowly if they aren't present in a high enough concentration and they may pass through the gut before causing any noticeable symptoms.

The minimum infective dose can also vary from individual to individual or due to environmental factors (for example, see this paper).