Public health authorities are terrified of measles for two reasons: (1) Measles is a respiratory infection that spreads as easily as the common cold. (2) Like HIV, measles damages the immune system. A case of the measles can kill directly, by causing severe inflammation in the brain. These deaths can come years after the child seems to have recovered from the measles. However, most of the deaths from measles result from immune suppression, which leads to other infections.
A measles virus infection starts off as an upper respiratory infection. Thus, an early case of the measles looks and feels like a common cold. It causes the sniffles, and it makes the person cough. With each cough, the infected person releases virus-laden droplets. These viruses can survive for a few hours outside the body. As a result, an unvaccinated child can catch the measles by sitting in a shopping cart or in a doctor's waiting room where an infected child had been two hours before. This is why nearly everyone who was born before 1957 has had the measles.
If the measles virus stayed in the upper respiratory tract, it would be no big deal. But the measles virus infection spreads to the immune system. At that point, the person develops a high fever and a rash. The person becomes really sick and is at risk for serious complications.
The measles is dangerous because it suppresses both parts of our immune system: the innate immune system and the adaptive immune system. The innate immune system is a general-purpose system that attacks infections of any kind. In contrast, the adaptive immune system provides a custom-tailored response to a particular germ that it has seen before. By suppressing the innate immune system, a case of the measles can lead to a deadly case of pneumonia. By suppressing the adaptive immune system, the measles can increase the person's risk of infection for up to 2 years.
The main workhorse of our innate immune system is a type of white blood cell called a macrophage (literally, "big eater"). Macrophages are supposed to swallow, kill, and digest germs that invade the body. Human macrophages do swallow the measles virus. But instead of killing it, they make more copies of it. So instead of fighting the measles virus infection, the macrophages make the infection worse. The infected macrophages die. As a result, the measles patient is at high risk for other infections, such as bacterial pneumonia.
The measles virus can also infect dendritic cells. The dendritic cells are scouts that find bits of foreign protein to show the to other cells in the immune system. The other cells then target that foreign protein specifically. Unfortunately, the measles virus infects the dendritic cells and keeps them from stimulating other cells involved in the adaptive immune response. This explains why a case of the measles lasts longer and is more severe than a common cold.
The measles infection can also kill off a lot of the body's B memory cells. The B memory cells are white blood cells that are supposed to carry the memory of other germs that the body has seen before. Thanks to these B memory cells, your body can remain immune to a disease for years after an initial infection. By killing off the B memory cells, the measles gives your immune system a form of amnesia.
During the rash phase of the illness, the measles virus can enter the brain. There, it can cause an infection. This infection can cause damage by producing inflammation. Inflammation of the brain is called encephalitis. The encephalitis can cause swelling. Since the brain is encased in the skull, the swelling causes pressure that damages the brain. The result can be blindness, deafness, mental retardation, or even death.
If the measles virus enters a baby's brain, it can remain there for years, leading to a condition called subacute sclerosing panencephalitis (SSPE). In this condition, the brain slowly turns to scar tissue. The baby seems to recover from the measles. But several years later, the child starts to show behavior changes and has trouble with walking. The condition always leads to coma and death. There is no cure and no effective treatment. SSPE occurs in children who catch the measles early, usually before they are old enough to be vaccinated. The only way to prevent SSPE is to vaccinate enough older children to prevent babies from catching the measles.
Since measles is highly contagious, we need to vaccinate nearly everyone to keep it from spreading. But if we vaccinate enough people all over the world, we will drive measles into extinction forever. After that, nobody will need to be vaccinated against measles.