In 2020 the topic “Communicable Diseases” gained great attention on Expertscape, for obvious reasons.

This month we have added warnings to that topic, to encourage visitors to choose a different topic. These warnings stem from the way the US National Library of Medicine (NLM) has defined “commnicable diseases” in its MeSH vocabulary. Expertscape relies completely on the NLM to define terms. We do not change their definitions. The best we can do is inform visitors when something appears amiss to us, as it does now, for two reasons.

The NLM's definition differs from the common meaning

NLM offers a “MeSH browser” that let’s anyone inspect MeSH terms. Thus, the MeSH browser page for communicable diseases tells us how the NLM defines the term.

The “Details” section of that page shows us the English-language definition for “communicable diseases,” and it is just what we would expect:

An illness caused by an infectious agent or its toxins that occurs through the direct or indirect transmission of the infectious agent or its products from an infected individual or via an animal, vector or the inanimate environment to a susceptible animal or human host.

But MeSH differs from an ordinary dictionary in an important way: it is hierarchical. The “MeSH Tree Structures” section of the NLM page shows this. It first defines “communicable diseases” as a sub-term of “infections” (as we would expect).

But notice that the MeSH page also specifies that “communicable diseases” has only four sub-terms:

  • blood-borne infections
  • communicable diseases, emerging
  • communicable diseases, imported
  • sexually transmitted diseases

This doesn’t seem right. It seems like a too-narrow characterization of communicable diseases. Where, for example, is pneumococcal pneumonia? That is certainly a communicable disease, but it does not fit into any of the four sub-terms.

You might argue that publications about pneumococcal pneumonia are “silently” placed into the communicable diseases bin, without fitting into an enumerated sub-term. We can show this is false with two experiments.

Experiment #1

Let’s pick a random publication about pneumococcal pneumonia… PMID 30290852: “An early increase in endothelial protein C receptor is associated with excess mortality in pneumococcal pneumonia with septic shock in the ICU”, published in 2018. Now let’s do two PubMed searches:

The first search returns our random publication, as we would expect, but the second search finds no publications satisfying its search. This proves that PubMed (and MeSH) do not consider pneumococcal pneumonia as a communicable disease. That’s just wrong.

Experiment #2

Another pair of searches confirms this finding. The MeSH browser defines “zoonoses” as: “Diseases of non-human animals that may be transmitted to humans or may be transmitted from humans to non-human animals.” Comparing this definition to NLM’s English-language definition of communicable diseases (above), it is apparent that all zoonoses are communicable diseases, but not vice versa.

We now perform the PubMed searches:

If all zoonoses were communicable diseases, then the two searches would return the same number of publications.

Thus, we conclude that the NLM’s definition of “communicable diseases” in the MeSH hierarchy does not match its English-language definition.

The NLM's definition changed markedly from 2020 to 2021

Regardless of the meaning for “communicable diseases,” it is clear that the definition changed in 2021 vs. 2020.

NLM updates its MeSH vocabulary at the end of every calendar year. Expertscape uses the new MeSH starting each January. Happily for this blog post, NLM operates separate MeSH browsers for both the current year (2021) and the previous year (2020). This makes it easy to determine if the definition changed.

The English-language definition for communicable diseases is identical in MeSH 2021 vs. MeSH 2020.

However, the hierarchical definition is markedly different. In MeSH 2020, communicable diseases has only two sub-terms: “communcable diseases, emerging” and “communicable diseases, imported.”

In other words, the definition for communicable diseases in 2021 has added two more sub-terms: “blood-borne infections” and “sexually transmitted diseasess.” This is a big change because, from 2010 through 2021 (Expertscape’s current window of interest), there have been 141,000 publications about blood-borne infections and 114,000 publications about sexually transmitted diseases

Infusions of these very large numbers of publications into 2021’s “communicable diseases” topic completely changes the meaning in Expertscape (and PubMed) as compared to 2020’s “communicable diseases” topic.

Thus, visitors should make no effort to compare this year’s results with last year’s – it’s apples vs. oranges.


Expertscape is best used for narrow topics, such as pneumococcal pneumonia, or Lyme disease, rather than broad ones such as “infections” or “communicable diseases.” In this day and age, it is not really possible to talk about experts in broad areas, just as it is not possible to have a person who is expert in all the areas covered in the Encyclopedia Britannica.

Indeed, narrowness of scope is inherent in the word “expert.” We therefore recommend that our users employ narrow searches wherever possible.