Conficker, a computer worm that began its spread though 200 countries in 2008 and remains active even today, uses a two-pronged approach to infect computers. Not only does the worm attack an operating system's inherent flaws, it also uses Internet malware techniques to exploit vulnerable networks. Equating HIV and this highly effective computer worm, researchers from University College London explain how HIV is able to spread through a patient's body in not one but two distinct ways: via the bloodstream and directly between cells.

In fact, it is this two-pronged approach or hybrid spreading that makes it possible for the incurable virus to continue attacking and infecting the body's immune system until a patient has developed acquired immune deficiency syndrome (AIDS).

“The model predicts that cell-to-cell spread becomes increasingly effective as infection progresses and thus may present a considerable treatment barrier,” wrote the researchers in their published work.

Finding the Right Model

Getting the right view on a situation is often the difference between continuing along forever with the same old problems or actually solving some difficulty and truly moving forward. If you’ve ever observed a child learning to speak, you would understand how, until a child makes the crucial connection between an object and a sound (a word), all the noise coming from other people's mouths are simply confusing noise. However, there is an exact moment when a light shines in her eyes and from that moment forward, she gets that particular sounds link to specific objects (or people), and from there she can absorb new words (and language itself) like a sponge.

Similarly, scientists need to create an accurate model in order to understand how something they cannot directly observe (disease, in this case) works. The right model links an unseen biological process with a logical prototype. The researchers of the new study, then, created their computer worm model to show how HIV progresses to AIDS.

Using detailed data from 17 patients, the researchers concentrated on how HIV infects CD4+ T cells. These white blood cells are commonly called helper cells in that their main function is to send signals to activate the body's immune response whenever they detect intruding pathogens — viruses or bacteria. These cells are the immune system's messengers, in other words, and not the actual fighters. As the virus attacks a patient’s T cells, their CD4+ T cell count falls and slowly the immune system loses function — this, in a nutshell, is how HIV becomes Acquired Immune Deficiency Syndrome (AIDS) over time.

The new model is important, then, because it proves HIV requires a second method of attack (just like a computer worm) to cause AIDS — spreading through the bloodstream alone would never be enough. Why not? Some areas of the body, such as the gut, have a really high T cell population, the researchers say, so the immune system would be able to fight off the virus. So, whenever HIV is surrounded by too many T cells, it switches infection techniques (just like a computer worm might do). Abandoning the suddenly ineffective bloodstream method, HIV begins using a cell-to-cell transfer mechanism to spread itself through a patient's body.

“HIV and Conficker have a lot in common,” Changwang Zhang, lead author and a doctoral student within UCL’s Computer Science department, told The Guardian. “They both use hybrid-spreading mechanisms, persist for a very long time, and are incredibly difficult to eradicate.” Going forward, Zhang said, scientists will be able to use the new model to evaluate strategies for treating and possibly eliminating the terrible virus from patients’ bodies.

Source: Zhang C, Zhou S, Groppelli E, et al. Hybrid spreading mechanisms and T cell activation shape the dynamics of HIV-1 infection. PLOS Computational Biology. 2015.