Part: 3 of 3
How viruses spread
Spreading from human to human
This section, like the rest of this virus saga, is about viruses that infect humans, and that spread directly from one person to the next. There are other types of viruses that infect plants or even bacteria, but those types are not relevant to this story, so I’ll leave them out.
The first section explained how a virus spreads from cell to cell. In summary, the virus infects one cell, and it is destroyed in the process. The cell then produces thousands and thousands of copies of the original virus. These copies then leave the cell, and they will infect new cells.
So far so good, but how does a virus get from one person to another? The answer is: by some way of body excretions.
This may be through sneezing. Many viruses cause symptoms like sneezing or coughing. This sneezing has a purpose (for the virus): it causes small droplets to be flung into the air. These droplets contain many viruses. If someone else breathes these droplets the viruses have a chance to infect a new person. The droplets may also land on a hand or on a doorknob. Someone else may touch this surface, get the virus particles on his own hand, and when this hand then comes into contact with his nose and mouth (for example, by eating), the virus may infect him.
Another way for a virus to leave the body is through diarrhea. Some viruses cause extreme diarrhea – the sole purpose of which is to provide a way for the virus to leave the body and thus to have a chance of entering new bodies.
After entering the new person’s body, the story of the previous section starts anew. Unless, of course, the new person has sufficiently strong lines of defense, as explained in the previous section.
When will an epidemic start?
What happens after the first person in a community is infected? Will the others be infected too?
This depends on the virus, on the circumstances, and on the strength of each person’s immune system.
- Soms viruses produce lots of “offspring” that leaves the body. Other viruses produce much less “offspring”.
- Some viruses are very good at penetrating our lines of defense, others are weaker.
- Once inside the body, some viruses multiply quickly, while others are much slower.
- Some persons are either immune to this virus, or they are not yet immune but their immune system quickly learns how to fight the new virus. These are the persons (like me) who only have a bit of slime in the throat and the nose, and who feel a bit tired, while others are severely ill for a week.
Will a virus spread? That depends on the above points. In the end, it’s all a matter of probability. A virus that produces a lot of “offspring” but is weak at penetrating a new person’s lines of defense may have the same chance of spreading as a virus that produces only few “offspring” but that is very good at penetrating the lines of defense.
If we would have perfect knowledge (we don’t), we could do the calculation like this:
- An infected person spreads X millions of new virus particles.
- Each of these virus particles has a chance of A percent to enter a new body.
- After entering the body, this virus has a B percent chance of penetrating all lines of defense.
- Having passed all lines of defense, infection is a fact. After infection, the virus has a C percent chance of finding itself in a body that has not yet built immunity against this type of virus, and that is slow enough to build up this immunity, so the virus can multiply first.
If “X times A times B times C” is high enough, the virus will, on average, infect more than one new person. As long as X, A, B and C remain unchanged within the community, the new person will again, on average, infect more than one person. And so on. The result is that ever more people get ill.
If, on the other hand, “X times A times B times C” is low enough, the virus will, on average, infect less than one person. Let’s consider an example. 100 persons are ill. They do infect other people, but on average each person infects less than one new person. Let’s say that in the “next round” only 70 persons are infected. Similarly, the third round sees only 49 persons infected. The fourth round has only 34 persons infected, the fifth round only 24, and so on. The disease slowly disappears from the population.
An example: the common cold
Let’s take the common winter cold as an example. This virus mutates quickly, so each winter we are confronted with a new strain to which we have no immunity.
- During winter, our behaviour changes. We stay closer together in heated rooms. This increases A, the chance that the virus can move from one person to the next.
- When outside in the cold, the mucous layer is thinner. This makes it easier for the virus to penetrate this layer. In other words, B is increased.
- The same low temperatures cause less blood to flow to our mucus membranes, carrying less immune cells to those membranes. This, too, increases B.
- At the start of the cold season, are immune systems aren’t trained yet to this strain of virus. Thus, C is high.
The result is that many people suffer from the common cold.
When the cold season ends, the opposite occurs. A, B and C all decrease. The virus will infect, on average, less than one new person. The infection with the common cold disappears from the population. Every now and then someone still catches a cold, but this will not spread to others.