# How many viruses does my immune system kill?

. . . in helping me get over the common cold? That is, when all is said and done, and I get over this cold, some little molecular parts of me will have destroyed a presumably large number of viruses? Does anyone know about how many? Does it vary much from disease to disease? Google is not helping me, and any info would be much appreciated.

[Carl Sagan]
BBillions and BBillions
[/Carl]

Couldn’t help myself.

Well here’s a quick and dirty response:

According to this paper (Warning: PDF), the researchers plotted a time course of viral titer in 20 healthy individuals over 8 days (Figure 1). Estimating really roughly from that figure, I figure that over those 8 days, the TCID50 = 10^3/mL. Considering there is approximately 5 L of blood in a human, that’s about 5E6. A rough mathematical estimation (Poisson distribution) translates that into approximately 3,500,000 ‘infectious units’, which is probably smaller than the number of physical viral particles by an unknown quantity.

I can’t be much more precise now, but I’ll ask around to some more knowledgeable people.

Actually, I was thinking billions too, if only because trillions seemed too high.

Thanks for this info Taenia spp. I don’t think I would have been abble to pull anything interesting off that chart.

Are viruses actually alive (can they be killed)? Or does one’s immune system just learn to inhibit their reproduction?

That’s engendered quite a lot of debate. Here’s a link that sums it up quite nicely.

So, how do we know when they’re dead? And if we can’t tell, is there any way to answer the OP’s question: “How many viruses does my immune system kill?”

It varies.

For example, in a very old paper (which I can’t figure out how to link to), the authors deduced that there is about one influenza virus per red blood cell in infected fowl (citation: DONALDH, . B. & ISAACSA,. (1954). J . gen. Microbiol. 10, 457-464.). Assuming this result applies to humans, and since humans have about 20 to 30 trillion red blood cells, it implies there are about 20 to 30 trillion influenza viruses in an infected individual.

As another example, in hepatitis C, a typical number of virus particles in an infected individual is around two million, but with a wide variation.

And, one more example, in HIV/AIDS, depending on the stage of the disease, there may be, as a very rough average, between 20 million and 2 billion HIV viruses in the body.

So, assuming my arithmetic is correct, there are huge variations in the number of viruses in a victim’s body depending on the specific disease being considered.

The immune system can destroy the infected cells (or the infected cells can go to autodestruct), which, although it can release millions of virus particles, can also cause their destruction if they’re not ready yet to be released. And by doing that, prevent spread to other cells.

Viruses are coated in capsules that can be recognized by the immune system and gobbled up by some of its components (macrophages, neutrophils). These cells don’t care for dead or alive. They’ll gobble up and destroy into tiny pieces whatever they take in and recognize as “bad”.

This is a popular question in popular science circles and the debates are interesting, but ultimately it’s not a very interesting scientific question. Most virologists may ask “Who fucking cares?”. You can define any arbitrary set of criteria that constitutes being ‘alive’ and viral particles will meet some of the criteria and not others. In the end, though, what’s much more interesting is that whether ‘alive’ or not, viruses have a broad range of replication and dissemination strategies that are fascinating in and of themselves to study.

Semantics aside, how can you tell if a virus is ‘dead’? Essentially, a virus is dead when it cannot propagate itself in the system you are studying. As an interesting aside, some viruses have such poor replication fidelity that up to 50% of newly formed viral particles have accrued enough errors in their genomes that they are functionally dead. Compare that with the near perfect replication fidelity our own genetic machinery achieves.