Measles virus

Measles is one of the most contagious infectious diseases, and remains a leading cause of death particularly among young children, especially in areas with low rates of vaccination.

Measles is also called rubeola, which can easily get confused with German measles which is also called rubella—similar sounding names but very different viruses.

Regular measles is caused by the measles virus, seriously, the species is the “measles virus”, of the genus Morbillivirus and family Paramyxoviridae.

The reason why this guy’s so contagious is that it’s airborne, and spreads via tiny liquid particles that get flung into the air when someone sneezes or coughs, and can live for up to two hours in that airspace or nearby surfaces.

If someone breathes in that air or touches a surface and then touches their eyes, their eyes, or their mouths, they can become infected.

Measles is so contagious that if one person has it, 90% of nearby non-immune people will also become infected.

Once the measles virus gets onto the mucosa of an unsuspecting person, it quickly starts to infect the epithelial cells in the trachea or bronchi.

Measles virus uses a protein on its surface called hemagglutinin, or just H protein, to bind to a target receptor on the host cell, which could be CD46, which is expressed on all nucleated human cells, CD150, aka signaling lymphocyte activation molecule or SLAM, which is found on immune cells like B or T cells, and antigen-presenting cells, or nectin-4, a cellular adhesion molecule.

Once bound, the fusion, or F protein helps the virus fuse with the membrane and ultimately get inside the cell.

Now this virus is a single-stranded RNA virus, and it’s also a negative sense, meaning it first has to be transcribed by RNA polymerase into a positive-sense mRNA strand.

After that it’s ready to be translated into viral proteins, wrapped in the cell’s lipid envelope, and sent out of the cell as a newly made virus.

Within days, the measles virus spreads through local tissue and is picked up by dendritic cells and alveolar macrophages, and carried from that local tissue in the lungs to the local lymph nodes.

From there it continues to spread, eventually getting into the blood and spreading to more lung tissue, as well as other organs like the intestines and the brain.

Now it typically takes 10-14 days from the time that the virus entering the body to the beginning of symptoms, and this is the incubation period.

Once symptoms start, we’ve entered the prodromal period which typically lasts around 3 days, and starts with a high fever and the 3 C’s —cough, conjunctivitis—or inflammation and redness of the white part of the eye, and coryza—swelling in the mucous membrane of the nose, essentially a stuffy nose.

1-2 days later comes the enanthem which is a rash on the mucus membranes, that looks like salt grains on a wet background.

These are called Koplik spots and are small white spots that are commonly seen on the inside of the cheeks opposite the molars.

After these initial prodromal symptoms comes the exanthem phase, which is where a red, blotchy, maculopapular rash spreads in a cephalocaudal progression.

In other words, the exanthem starts at the head (or cephalo), and spreads to the extremities or ends of the body (or caudal).