Worldwide HIV/AIDs Epidemic Statistics
Worldwide HIV/AIDs Epidemic Statistics

Viruses (and other pathogens, of course) can become more or less virulent over time.  This is generally a side effect; the virus couldn’t care less whether it kills its host or not, but it does care deeply about how well it is transmitted to new hosts, because if that’s not efficient then the virus will die out.  Sometimes viruses can be better transmitted by reducing virulence — for example, if the host survives longer, it may contact more new victims.  Sometimes viruses can be better transmitted by increasing virulence — if it’s transmitted by the fecal/oral route, then causing more diarrhea and vomitting may improve transmission.

As viruses enter new populations, such as when the virus jumps from one species to another, they may cast around looking for optimal strategies; their virulence and transmission may work well in one population but not in another.  One such virus is HIV, which entered the human population something like a hundred years ago but didn’t really explode until much later.  There’s been a lot of interest — for obvious reasons — as to whether HIV is becoming more or less virulent, or whether it hasn’t changed at all, since the HIV epidemic started about 30 years ago. It’s hard to come up with a solid theoretical prediction either way, partly because HIV transmission isn’t necessarily linked to virulence in an obvious way.

Unfortunately the answers to the question have been all over the place.  I summarized a dozen or more studies, last time a paper on this came out, by saying that “Experimental evidence has pointed in all directions — some suggests that HIV is becoming less virulent, some that it is becoming more virulent, and some says it’s staying the same.”  The study I was talking about 1 concluded that HIV was not changing in virulence:

Thus, the results of this study do not support the hypothesis that there has been any important change in the virulence of HIV-1 over this time period in this cohort. 1

HIV budding from a lymphocyte
HIV budding from a lymphocyte

A new study2 reaches a different conclusion.  They found that people infected with HIV recently seem to be progressing more rapidly toward AIDS, because they have lower CD4 T cell counts on first presentation. 3

The decrease in the post-seroconversion CD4 cell counts occurred early in the epidemic, with stabilization since the advent of HAART. These data may provide an important clinical correlate to studies suggesting that HIV may have adapted to the host, resulting in a more virulent infection. 2

There’s an excellent accompanying editorial4  that outlines the background of the question, quickly summarizes the theoretical problems in making predictions, and offers a possible different interpretation:

The increased circulation of more-aggressive HIV subtypes could also explain the results of studies that show increased virulence over time. In this case, however, the apparent increased virulence would not be the consequence of a selection process but only an effect of a greater representation of more-virulent subtypes … 4


  1. Herbeck, J.T., Gottlieb, G.S., Li, X., Hu, Z., Detels, R., Phair, J., Rinaldo, C., Jacobson, L.P., Margolick, J.B., Mullins, J.I., Tripathy, S. (2008). Lack of Evidence for Changing Virulence of HIV-1 in North America. PLoS ONE, 3(2), e1525. DOI: 10.1371/journal.pone.0001525[][]
  2. Crum-Cianflone, N., Eberly, L., Zhang, Y., Ganesan, A., Weintrob, A., Marconi, V., Barthel, R., Fraser, S., Agan, B., & Wegner, S. (2009). Is HIV Becoming More Virulent? Initial CD4 Cell Counts among HIV Seroconverters during the Course of the HIV Epidemic: 1985–2007 Clinical Infectious Diseases, 48 (9), 1285-1292 DOI: 10.1086/597777[][]
  3. This is a highly simplified summary! They did do a lot more controls and tests.[]
  4. Has Human Immunodeficiency Virus Become More Virulent? Maria Dorrucci and Andrew Phillips. Clinical Infectious Diseases 2009;48:1293-1295 DOI: 10.1086/597778[][]