Measles is caused by infection with the rubeola virus. This is a highly infectious virus spread by airborne droplets from an infected person.
The measles virus is a paramyxovirus belonging to the genus Morbillivirus. The virus is 100–200 nm in diameter and has a core of a single stranded RNA. The virus is closely similar to other viruses like rinderpest and canine distemper viruses.
It has two membrane envelope proteins that are important for pathogenesis of measles in humans. These proteins are the F (fusion) proteins that are responsible for fusion of virus and host cell membranes. This fusion further leads to penetration of the virus and eventually breakdown of the RBCs (hemolysis) in the host.
There is also the H (hemagglutinin) protein that helps the virus to be adsorbed into the host cell. As an antigen there is a single type of measles virus. The H glycoprotein, may have some amount of variability but this does not cause changes in the virus or affect the efficacy of the measles vaccine that works against it.
The measles virus remains a delicate virus and is rapidly inactivated by heat, light, acidic pH and by chemicals and enzymes such as ether and trypsin. It can survive for less than 2 hours in the air or on objects and surfaces.
The measles virus is present in the millions of tiny droplets that come out of the nose and mouth when someone with measles coughs or sneezes. A healthy individual who inhales these droplets can catch the infection.
The infection can also be spread if the droplets have settled on a surface and a healthy individual touches the surface and then places the contaminated hand over or near his or her nose and mouth.
Once the measles virus enters the body, it multiples in the back of the throat and lungs and starts to spread throughout the body including the skin and respiratory system. The capability of infecting others lasts for two to four days before the rash appears and for about five days after it appears.
Measles can affect all age groups of individuals. Anyone who has not had measles before and has not been vaccinated can be infected. Cases of re-infection are rare since the body builds immunity to the virus.
Those at a high risk are: