Syphilis
I have read a fair ampount on the subject of Syphilis
The evidence all points to it being derived from the
Tropical Disease - Yaws.
This has been endemic in Africa ( amongst other places)
for thousands of Years.
In that form it was simply a skin Disease - which was
passed-on largely by skin contact, between scantily
clothed people in hot climates. This was particularly
between scantily clothed children.
When Europeans contacted these Tribal Peoples, some
may have caught it, and brought it back to Europe.
However, people in Europe are usually clothed -
and so this Disease had to evolve, to find another
means of passing-on.
At first it was passed largely by Kissing - in an age
when even men often kissed each other on the cheeks,
and ladies were kissed on the mouth.
However, by passing to a more "internal" form - Syphilis
had already started to change from the Yaws form.
It gradually evolved to be passed by contact with mucus
and thin membranes - including the sexual parts.
Later, this became a Sexually Transmitted Disease -
which often resulted in detectable Lesions on or in the
sexual parts - however, it retained its aspects of being
a Skin Disease in that often part of the face or sexual
parts would begin to be "eaten away" visibly.
This was its evolved form by the 1800's. - but the worst
aspects were its more "internal" aspects.
In what were called cases of "third stage" Syphilis -
the disease could attack the lungs or digestive system,
causing serious damage.
Even worse in some ways was when it attacked the
nervous system, causing severe or even fatal paralysis,
or when it attacked the Brain, causing Dementia
and eventually Death.
No true effective cure was ever really found - some
compounds of Mercury were used to treat first and
second stage Syphilis - but with limited success.
Often the treatment was worse than the Disease.
The only effective Treatment ever developed was
when Penicillin was discovered by Fleming, which
worked well against Syphilis in all its stages
( providing irreperable damge had not been done ).
Later, other Anti-Biotics were used to effectively
combat Syphilis.
Concerning your possible Airborne Syphilis -
the problems are that this kind of Mutation would
take an appreciable time.
IMHO perhaps hundreds of years.
It took that long to mutate into a form which could be
transmitted mainly by sexual contact ( but still also by
Oral Sex and Kssing ).
There is no evdience of any form which could survive
outside of the human body - you cannot catch syphilis
by touching soiled underclothes. I would not recommend
mouth contact with soiled undergarments though.
This idea sounds a little weak