In the book I’m reading there is a contagious illness that disfigures children but won’t kill them; in adults it’s a death sentence.
What diseases/conditions exist in real life that have vastly different effects on children vs. adults.
ISTR that chicken pox is a much bigger deal in adults than it is in kids.
Getting your tonsils out as an adult can be a big deal and a carries significant risk of complications including sudden hemorrhaging and possibly death. It is usually considered a routine and rather minor surgery for children.
Adult males with mumps can have it settle in their testicles and scrotum, which can affect fertility.
Lukemia? Isn’t the prognosis for a child with lukemia better than for an adult? Not that it’s just an inconvenience, but…
As Max Klinger said: “If you get it as a kid, you don’t get it as an adult. If you get it as an adult, you don’t get kids.”
The prognosis for the majority of childhood cancers is significantly better than the average prognosis for adult cancers. Of course, most of those survivors are middle-aged or younger and it’s still not known if they will have problems as they get older.
Vice versa: Reye’s syndrome. Children who take aspiring can develop it, while aspirin is comparatively harmless in adults, and adults contracting Reye’s is rare.
Honey can give botulism to infants. Adults can eat honey harvested by the ancient Egyptians to no ill effect.
And many congenital STDs may not harm the mother much, but could cause brain damage in the fetus. These have a chance of increased risk of death.
I’m surprised nobody’s mentioned chicken pox. Or has that myth been debunked recently?
It’s in the OP. 
Some cancers wouldn’t occur in childhood (of at least be rare): skin, breast, uterine, testicular.
:smack: This is what happens when people don’t read the entire post before commenting
So, some good examples then … ummm ears start to steam … Oh yeah, but about things like scoliosis and other physical deformations? They’re much more treatable in children since their bodies can be guided into growing into the proper configuration.
Impotence/ED: irrelevant (and unheard of) in kids, a major problem for middle-aged men (if the ads are to be believed :P)
Male pattern baldness: Even people that have it are rarely affected before puberty.
I believe the Spanish flu had a somewhat peculiar mortality / age distribution pattern, taking a very high toll of young adults compared to children and elderly people. Quite the opposite of most flu epidemics.
Poliomyelitis.
In very young children (like, under 6 weeks or so, IIRC) it’s typically no worse than a bad cold. It might leave some long-lasting effects, like scoliosis of varying degrees. (A doctor once speculated that my scoliosis might be the result of infantile sub-clinical polio.)
Once upon a time, before parents were too gung-ho about hygiene, and babies were allowed to crawl around on the floor or in the dirt (rather than being kept in cribs), polio was not the horrific terror that it’s known for now. One popular theory holds that nearly all babies caught it as infants, with the parents typically thinking it was just a bad cold. This left the babies with immunity, so it never became an epidemic among older children or adults. When parents began keeping their babies in cribs, off the floor, then babies didn’t get this immunity. Then polio became the scourge that it is known for, as more children and adults began to get it when they were older.
(I don’t have any cites for this at my fingertips, and I’ve also read occasional opinions that the above is total horse shit. But apparently, it’s at least a known theory of why polio became the disaster that it did, when it did.)
Pertussis (whooping cough) can kill babies and small children, but is usually a much less serious disease in older children and adults.
Rubella is often subclinical, meaning that people often don’t even know they had it, but if a woman contracts it during the first trimester of pregnancy, it can cause serious problems in the baby, the most common being deafness.
Okay, here’s a cite for the polio / hygiene theory. (It says polio tends to be rather benign in children up to 6 months, not 6 weeks.)
Wikipedia: History of poliomyelitis
Which is actually a plot point in a certain Agatha Christie novel, though with some form of retardation rather than deafness.
Falling, breaking bones usually have quick recoveries for children–but are extremely hard to recover for the elderly.
Deafness is the most common problem, but prenatal rubella can also cause blindness, heart defects, and mental retardation.
Lassa fever, thought to be a “new” disease ca. 1970, is now believed to be, for most people in western Africa, just another childhood disease that causes youngsters to run a high fever for a few days. In adults, it has about a 25% fatality rate.
Taking this slightly off-topic, but I really would NOT recommend getting mumps as an adult. A friend of mine got it last year and… let’s just say that he had an orange in his pants for a while