Measles is an infectious disease caused the
measles virus, and it has been around for a very, very long time. According to CDC, one of the first
written accounts of measles popped up in the 9th century. The thing about measles is that it is
incredibly contagious. It’s considered to be one of the most common, and most unpleasant, viral
infections in existence. Historians believe that transmission of measles increased dramatically
as trade routes expanded and worldwide travel became more popular. And even though measles
is notorious for causing childhood disease, it can also cause severe symptoms in adults.
In fact, there were over 21,000 reported cases of measles on both sides of the Civil War
in the 1860s, resulting in over 500 deaths. By the arrival of the 1900s, it was expected
for nearly all children to get measles by the time they were 15 years old, and those
cases usually involved rash, high fever, cough, inflammation of the nasal passage or
conjunctivitis. Millions of people in the United States were infected with measles each year, some
of whom required hospitalization, and a smaller number suffered encephalitis as a complication,
a word which refers to a swelling of the brain. With a brief history covered, let’s talk more
about the virus itself. The measles virus is part of the Paramyxoviridae family, and within
that, the Morbillivirus genus. The other two genera in this family are Paramyxovirus, which
includes the parainfluenza and mumps viruses, and Pneumovirus, which includes respiratory
syncytial virus, or RSV, and metapneumovirus. Paramyxoviruses are made up of negative-sense,
single-stranded RNA, that’s packaged in a helical nucleocapsid and surrounded by an
envelope. In the case of paramyxoviruses, the envelope is pleomorphic, which means it can
vary slightly in size and shape. Paramyxoviruses are similar to orthomyxoviruses like influenza,
but they tend to be larger and don’t have the segmented genome that influenza viruses do.
Within that pleomorphic envelope surrounding paramyxoviruses, there are two glycoproteins
that facilitate viral replication within a host. One of these glycoproteins is a viral attachment
protein that helps the virus attach to the host cell to begin with. The other is a fusion
protein that supports the process of the host cell membrane and viral membrane fusing together,
which allows the virus to enter the host cell. The process of fusion involves a series of intricately
controlled steps carried out by viral proteins, the blueprints for which are packaged neatly
into the viral structure. The fusion process basically shepherds the virus into the host
cell to start replicating in the cytoplasm. As we mentioned, measles is highly contagious.
The measles virus is spread from person to person through respiratory droplets, so all it takes
is inhaling droplets that an infected person has exhaled, talked, coughed, or sneezed into the air.
Once the virus is inhaled, it sets up shop in the respiratory tract, where it replicates for a bit
before moving on to spread through the lymphatic system and blood. The virus is thereby widely
distributed throughout the body, infecting the respiratory tract, blood vessels, urinary tract,
lymphatic system, and the central nervous system. Ultimately, this multi-system attack causes
the characteristic maculopapular measles rash, along with fever, cough, and conjunctivitis.
Compared to other viruses, the symptoms of measles are fairly easy to recognize, so laboratory tests
aren’t always needed for an official diagnosis. Most people recover from measles and
develop life-long immunity to the virus, though some may develop pneumonia or encephalitis,
which can be deadly complications. It’s also worth mentioning that there is only
one serotype of the measles virus, and as luck would have it, it infects only humans.
To put things in context, measles is considered to be one of the five classic childhood diseases
along with rubella, roseola, fifth disease, and chickenpox. These diseases are also called
exanthems, a word that refers to an eruptive disease or symptomatic eruption of disease.
Since the development of measles vaccines, and especially the live attenuated
measles vaccine developed in the 1990s, fewer than a thousand cases have been reported
in the United States. The measles vaccine is often combined with mumps and rubella, which is
called the MMR vaccine, or combined with mumps, rubella, and varicella, and referred to as MMRV.
Worldwide, however, measles is still one of the most prominent causes of disease and death in
places where the population is unvaccinated.
