It is because of the way the body responds to the virus and what tissues the virus infects. The virus that causes AIDS for example, affects different tissues and doesn't show the same way as measles does.
measles
They are not hidden, when you get measles, you would get symptoms right away.
The vaccine preventing measles is for measles only. Other diseases with the same symptoms are much less serious and don't need that much immunity
There is more than one type of measles (plus 21 strains), rubella, smallpox, and chickenpox all have similar signs and symptoms.
German measles is somewhat a higher type of measles infection that usually affects adults that have been previously affected with measles. This is still a communicable disease but has a different characteristic than the latter and has more severe symptoms.
Measles symptoms can take up to 14 days to show. They are fever, cough, runny nose, pink eye, small blisters in the mouth and large red blotchy marks on the skin.
There is no specific treatment for measles mainly because it is a viral infection and the management is usually of the symptoms. Sometimes antibiotics are given to manage bacterial complications accompanying measles such as pneumonia. It is best to get vaccinated against measles with the MMR vaccine (measles, mumps, rublla) which will prevent you from having measles in the future.
Called encephalitis, this can occur up to several weeks after the basic measles symptoms have resolved.
why older people weren't affected by measles
Measles symptoms invariably include fever, together with at least one of these, cough, runny nose, swollen eyelids, dry hacking cough, watery eyes, sneezing, and sensitivity to light.
An acute specific disease with a dusky red cutaneous eruption resembling that of measles, but unattended by catarrhal symptoms; -- called also German measles.
There are two types of measles. Although they share some of the same symptoms, they are caused by different viruses: 1.) Standard measles, sometimes known as red measles, or hard measles, is caused by the rubeola virus. 2.) German measles, also known as rubella, is an entirely separate illness caused by the rubella virus and is usually a milder infection than standard measles.