The short answer: like pretty much any other well-equipped virus.
So, measles is again on the march in the United States and elsewhere in the developed world. It hasn't reached the "fact of life" status it held during the virus's pre-vaccine party-times, but nearly 100 cases of measles had been reported and recorded in the United States in January, mostly stemming from a single outbreak traceable back to a single day at DisneyLand. The afflicted have been almost uniformly unvaccinated.
But what is measles exactly? The virus's consequences are about the most uncomfortable-looking thing imaginable—a full-body mat of lesions and blisters—but how does it go about achieving such gruesome effects?
The virus hails from the same family, paramyxoviridae, as deadly viral pneumonia and the since-eradicated rinderpest, an epizootic (animal) disease blamed for the deaths of many millions of cattle and wild animals. Not bad for a molecular structure so simple that it barely qualifies (if it does at all) as being alive in the first place. More so, it's like a tiny machine, a biological landmine packing genetic material instead of explosives.
The measles is transmitted via aerosolized droplets and it doesn't take much. Droplets can hang around in closed environments for up to two hours, but more likely you'd catch it in the usual way of being coughed on or near by someone carrying the virus.
The "secondary attack rate" for people exposed to the virus is upwards of 90 percent. To put that into perspective, the secondary attack rate for the flu is between 10 and 20 percent and around 3 percent for full-blown tuberculosis, while the risk of HIV transmission is roughly around 1 percent per exposure (a rough average of exposure types). Using the r_0 value for measles—how many secondary infections are likely to result from a primary infection—it's about the most infectious thing going with a value of 18. HIV is down at 4 infections per primary and the mumps comes in closest at 10.
(Note however that r_0 values are dynamic, and those are just some recent estimates.)
Basically, the virus finds its way to some healthy cell and chemically binds to it using one of three surface glycoproteins, which are the proteins projecting from cells in order to mediate cell-to-cell interactions. The virus then fuses with its target cell and starts making new genetic material. This becomes the viral mRNA that will go on to "hack" the target cell, which will in turn go on to make more viruses and the process repeats again and again while you start to feel like shit as the infection builds.
With measles (and most viruses you're likely to catch), the dominoes start to fall within lung tissues, where they prey on immune system early-warning defenders: macrophages and dendritic cells. Next, its viral armies move deeper into the immune system, invading lymph nodes and and latching onto white blood cells, who then go on to spread the virus around the body. "The spleen, lymph nodes, liver, thymus, skin, and lungs are eventual destinations for the virus," David Shultz writes for Science.
"In rare instances (about one in 1000 cases), the virus can cross the blood-brain barrier and cause dangerous swelling of the brain," he adds.
Of course, it's not the hacking cough, persistent pink eye, or extreme fever that really characterize the measles. It's the brutal all-over rash, a dense collection of flat red splotches that can, in some cases, cause a viral reinfection leading to encephalitis, where either a weakened immune system mistakenly targets the brain or the brain is directly infected by a virus. It can cause neurological problems or even be fatal.
The rash isn't actually unique to measles. Specifically, it's what's known as a maculopapular rash; you'd wind up with something similar with Ebola, scarlet fever, secondary syphilis, or, in some people, the right amount of heat exposure.
It's inflammation. As the virus cruises around your body, it infects the tiny capillaries at skin-level, which the immune system responds to by releasing nitric oxide and histamines, which hurt the viruses but also the surrounding tissues. In turn, the tissues become inflamed, with the result being swelling and damage. A rash.
Really, this is the general inflammatory effect of viral infections: infect and watch the body turn on itself in the process of defending itself. In the extreme, the effect is a cytokine storm, where the immune system goes all out against some invader.
Suddenly, the body is full of signalling chemicals, every one telling it to produce some antibody and/or toxin and/or new signal. In Ebola, the storm is so powerful that the resulting inflammation begins to wreck the blood vessels themselves, hence the bleeding. Meanwhile, cells are dying all over and necrotic tissue accumulates, making for an overall very bad scene. Fortunately, measles doesn't have quite that effect.
Still, measles isn't something we want loose in the world—or something that even needs to be loose in the world at all.