Measles virus shares many characteristics similar to Ebola virus. It is made up of a single strand of RNA (negative strand) that is surrounded by proteins and a membrane (see measles virus in Table 3). Unlike Ebola, which requires direct contact with body fluids, measles infection occurs when a person inhales virus-laden droplets exhaled from an infected person. The virus particles enter the lungs and, like Ebola, attack immune cells, the body’s first line of defense against infectious agents. The virus binds to the surface of the cell and is taken inside the cell where it releases its genetic material and enzymes needed for virus growth. Inside the cell, the virus makes many copies of itself through steps characteristic of negative-strand RNA viruses:
- Viral RNA polymerase transcribes mRNA from the negative-strand viral RNA.
- Using the host protein synthesis machinery, the mRNA directs the synthesis of viral structural proteins and enzymes.
- Viral RNA polymerase uses the negative-strand RNA also as a template for the production of a full-length positive-strand RNA as a step in the production of new virus particles (progeny).
- Viral RNA polymerase uses the positive-strand RNA (step 3) as a template for the production of full-length negative-strand RNA genomes.
- The negative-strand RNA genome, the viral RNA polymerase, and the structural proteins join together to form enormous numbers of viral particles.
- The virus exits the cell by budding from the host cell, picking up a coating of host membrane as it leaves.
- The host cell then disintegrates because the virus has hijacked its protein making machinery and therefore can no longer survive
The infected cells, laden with viral particles, migrate from the lungs to the lymph nodes, infecting more immune cells and spreading the virus to other sites in the body, including the spleen, thymus, and skin. The skin rash characteristic of measles is the result of infection of cells in the skin. In some cases, the virus can reach the brain, where it may cause permanent brain damage.
After several days, the viral-infected cells reach the nasal passages, infecting epithelial cells that line the upper respiratory tract and producing large numbers of viral particles. Infected individuals release clouds of virus-laden droplets from their noses, trachea, tonsils, and lungs. These droplets can remain infectious on surfaces for several hours and can move via air currents to the next susceptible host. This mode of transmission makes the measles virus highly contagious. Besides a rash, other symptoms of measles include coughing, runny nose, and red watery eyes that result from virus infection and cell death and a high fever, a sign of the immune system doing battle. In some instances, measles can lead to more serious symptoms and even death. Prior to the development of an effective vaccine in the early 1960s, 7 to 8 million children died of measles worldwide every year. By 2014 that number was reduced to 145,000.
Because immune cells are a major site of virus reproduction, a bout of measles can leave the victim with an immune system that is somewhat disabled, making that person vulnerable to other infections. Recent studies have indicated that following an infection, a child’s immune system can be weakened for up to three years, leaving the child susceptible to infections that a fully functional immune system would normally fight off.