HIV is one of the best-known viruses in existence.
Despite this, compared to some of the other viruses we’ve covered, our knowledge about
HIV is relatively new.
Human immunodeficiency virus began circulating and infecting humans long before we knew what
it was and where it came from, as with most diseases in human history.
But HIV in particular is considered to be one of the most devastating infectious agents
to emerge in recent history.
The story begins in the early 1980s when a mysterious disease popped up quite suddenly.
This was recognized due to increasing numbers of unusual opportunistic infections and rare
cancers that could be transmitted sexually, through injection drug use, from pregnant
women to their babies, or through blood transfusion.
CDC identified the disease as “acquired immunodeficiency syndrome”, or AIDS, but
it wasn’t until several years later that scientists figured out that a retrovirus was
causing AIDS.
It took years, a few name changes, many lost lives, and a mountain of hard work from many
researchers, but we now know that human immunodeficiency virus is a retrovirus that attacks the body’s
immune system, and can lead to AIDS if not treated.
Now we have heard the term retrovirus before, but what are they exactly?
These viruses are enveloped, positive-strand RNA viruses, and they tend to have unusual
morphology and replication strategies.
More specifically, retroviruses have a capsid containing two copies of a positive-strand
RNA genome.
Back in the 1970s, scientists David Baltimore and Howard Temin showed that the retroviruses
encode an RNA-dependent DNA polymerase, which is also called reverse transcriptase.
Essentially, this reverse transcriptase enzyme allows the virus to make copies of DNA from
RNA.
At the time, the “central dogma” of molecular biology as it is sometimes colorfully referred
to, was the idea that genetic information is passed only from DNA to RNA, and then from
RNA to protein, just as we learned when we studied transcription and translation in the
biochemistry series.
Baltimore and Temin’s finding demonstrated that this was not exclusively the case, and
their discovery ended up earning them a Nobel Prize.
Among the human retroviruses, there are three subfamilies: Oncovirinae, which is associated
with cancer and neurologic disorders, Lentivirinae, which includes HIV-1 and HIV-2, and Spumavirinae,
which doesn’t cause clinical disease, but does cause visible changes in infected cells.
The retroviruses are classified by a few factors: the disease they cause, host range, tissue
tropism, virion morphology, and genetic complexity.
But let’s focus specifically on HIV.
Human immunodeficiency virus primarily infects CD4 T cells, which are white blood cells that
fight infection, but the virus can also infect macrophages, which are another type of white
blood cell.
The envelope of HIV-1 has proteins that form spikes on the virion’s surface, and it’s
these glycoproteins and transmembrane proteins that attach to a host cell and help the virus
get inside.
In the case of CD4 T cells, the virus causes lytic infection.
With macrophages, the virus causes persistent low-level productive infection.
Since the virus is killing one type of host cell altogether, the CD4 T cells, in order
to make more copies of itself, it dramatically affects the immune response and ultimately
leads to immunosuppression.
There are multiple different pathways that the virus can take to trigger disease, depending
on what cells it infects and where they are, and they are all very, very complicated.
But the overall gist is this: HIV is very skilled at hiding from different components
of the immune system, and one of the major factors contributing to this is its ability
to repeatedly undergo mutation.
Let’s talk about the actual disease HIV causes.
Stage 1 of HIV is an acute HIV infection, where an infected person might have flu-like
symptoms, or might not show symptoms at all.
During Stage 1, there is a very large amount of virus in the blood, and an infected person
is very contagious.
Stage 2 is chronic HIV infection, which is sometimes called clinical latency or asymptomatic
HIV infection.
By the end of Stage 2, the amount of HIV in the blood, or the viral load, goes way up,
while the CD4 cell count goes down.
The most severe phase of HIV infection is Stage 3: Acquired Immunodeficiency Syndrome,
or AIDS.
By this point in the disease progression, someone infected with HIV has such a badly
damaged immune system that they are more likely to get severe cases of opportunistic infections,
and without treatment, people with AIDS typically survive about three years.
Even though there is no cure, there are medications available to keep infections from developing
to Stage 3.
So where did HIV come from?
Scientists have determined that HIV infection in humans originated from a type of chimpanzee
in Central Africa.
The chimpanzee version of the virus, called simian immunodeficiency virus, or SIV, got
passed to humans as early as the late 1800s.
Over the course of the decades that followed, HIV slowly spread across the world, and we
believe that HIV has existed in the United States since the 1970s.
Thinking about transmission, HIV can not be transmitted by sharing food or drink, through
pets or insects, by air or water, or through sweat or tears.
There are very specific activities that can transmit HIV, including sexual activity, and
needle or syringe use, though the virus can also be spread from an infected mother to
their child during pregnancy, birth, or breastfeeding.
As of 2018, CDC estimates that 1.2 million people in the United States have HIV, with
just under 40,000 new diagnoses that year alone.
Worldwide, though, CDC estimates that there were 1.7 million new cases in 2018 and about
37.9 million people living with HIV.
When it comes to prevention, using condoms can help, and there are a few relatively new
prevention medications available, such as pre-exposure prophylaxis, or PrEP, and post-exposure
prophylaxis, or PEP.
For those that have HIV, the most effective treatment is antiretroviral therapy, or ART.
If an infected person takes this medication as prescribed, they can stay healthy and have
a very low risk of transmitting the virus to a partner.
So while tremendous progress has been made in treating HIV since the 1980s, we do still
have a ways to go before we are truly free of this virus.