I know your question specifies venereal diseases from prior to the appearance of syphilis in Europe, but the time period you've identified in your comment, 1400-1600, would have been the peak time for "the pox." It was supposedly introduced in 1494 and spread northward through Europe from Naples, although its origins are a matter of controversy--so much that contemporaries blamed it on one another with appellations like the "Neapolitan disease" or the "French disease." Prostitutes were also commonly blamed for its spread. Some historians believe it may have been introduced by returning ships from the New World, while others contend that it may have been already present in Europe as early as the classical period, but reached epidemic proportions in the sixteenth century. In any case, contemporary accounts suggest that syphilis upon its appearance in Europe was much more virulent and fast-acting than it is today--so much so that it is debated if the diseased called "the pox" is in fact an equivalent to what we call syphilis at all. Thus contemporary descriptions of pox-stricken people with missing noses and disfigured faces crowding the streets of London and Paris. So to answer your question, people would have deteriorated much more rapidly of syphilis--if it was in fact syphilis--than they do today.

As for venereal diseases that existed prior to the sixteenth century, leprosy in large part informed how people later understood the onset of the pox. Similarly, it is unclear what relationship leprosy has to modern diseases and is also a matter of debate. Many cases that describe leprosy could have in fact have been syphilis, as both produce horrific skin lesions. Most importantly, however, people understood both these diseases first as the result of a moral contagion and only over time made the specific connection to genital contact. In the twelfth and thirteenth centuries and onward, leprosy was believed to be a result of moral corruption or a divine punishment, although perhaps also as a result of illicit sex. People afflicted with leprosy were often expelled from cities and housed in leper colonies, because they were considered morally unclean and contagious. Known prostitutes periodically faced the same treatment in the medieval and early modern periods during periods of outbreak because of their association with disease.

Once people began to realize that the pox and leprosy were spread by illicit sex in the sixteenth century, they did make some efforts to avoid contracting it. The visible marks of venereal disease were well known and a subject of shame and ridicule. Consequently, condoms made of animal intestines were invented in the sixteenth century, primarily to protect men when having sex with prostitutes rather than as a birth control method. The first known description of a condom is in fact included in Gabriello Fallopio's 1564 text on syphilis. Although documentation is lacking, it also seems that a good deal of illicit sexual activity in the early modern period involved non-penetrative sex. Samuel Peyps, for instance, recorded "caressing" and masturbating onto several of his maid-servants rather than having sex with prostitutes, presumably out of fear of contracting a venereal disease. How commonly methods like this would have been used strategically is a matter of uncertainty though.

I hope this begins to help answer some of your questions. Perhaps someone else can help fill in more on venereal disease and how it affected the perception of prostitutes, as I know less about this.