All of them knew that during the last years, Schubert didn't feel well from time to time and his condition didn't seem different from any other before, they didn't get worried; Nobody warned close friends such as Moritz von Schwind (who lived in Munich at that time), and at the Bruchmann's home, where Schubert was so much loved, his friend's illness didn't darken the wedding preparation for Justine, their daughter. However, suddenly, on November, 17th Schubert worsened and became delirious; the priest barely arrived in time to give him the last rites. As we know, Franz died on the 19th, early in the afternoon, and his friends heard about his death at the wedding party.
The doctor who treated Schubert determined nervenfieber as the cause of his death; that's how typhoid fever was called at that time. There wasn't any epidemic in Vienna but isolated cases were frequent, as the illness was spread by drinking contaminated water and water wasn't always safe enough; it wasn't unusual either that patients died (in fact both Schubert's mother and brother Ferdinand died of nervenfieber). So, according to the oldest versions, Schubert didn't die of syphilis but of typhoid. Indeed, the symptoms of Franz' illness matched those of typhoid and one detail reinforces this theory: the hypochondriacal Schober didn't visit him, afraid of being infected.
Nevertheless, some authors allege that the term nervenfieber was also used, more broadly, to describe any other condition that had similar symptoms (high fever, delirium, loss of consciousness or convulsions) and wasn't more specifically described. Syphilis, in some cases, had those symptoms. Based on this, some experts recently said that Schubert died of syphilis, and that the first versions talked about typhoid partly out of ignorance, but also to protect his memory.
The first stage of syphilis presents chancres a few days or weeks after infection, it's the warning signal (and now that we have antibiotics, the end of the disease); secondary syphilis, relatively mild, has some symptoms very similar to those of flu and others more evident, such as skin lesions and hair loss. After that, the symptoms disappear and it seems that the patient is cured. This latent phase may last from a few months to decades until tertiary syphilis arrives. Sometimes this last phase is relatively benign but it usually affects the nervous system (it's the case of Hugo Wolf, for example) or the heart, and death comes in a few years. Schubert was not in any of these situations, but syphilis could also affect other organs like the digestive system or the liver and cause nervenfieber. So, given that Schubert had syphilis, consequently, he died of syphilis. Statistics are against this theory though: on average, tertiary syphilis manifests fifteen years after primary syphilis, and Schubert had been infected only five years before. In addition, if he suffered from tertiary syphilis, it lasted a really short time. There is another point to be considered: the doctor who visited Franz wasn't the regular, because he was sick. Instead, a syphilis expert went. Was it by chance, or was Ferdinand who summoned him because of his expertise? And, if he was an expert in syphilis, why would he certify vague nervenfieber as the cause of his death instead of syphilis, unless, it really was nervenfieber, typhoid fever?
Are you thinking it's pretty confusing? There is still a third theory: death by mercury poisoning. The syphilis treatment was based on this toxic metal and have many side effects; most likely, those health problems that Schubert have had during his last years were due to mercury. In certain cases, depending on the dose, treatment duration and tolerance, poisoning was so severe that the patient finally died and, yes, the symptoms could be very similar to those of Schubert.
As you can see, there isn't any concluding theory. As I told you at the beginning, we only know for certain that Schubert death was unexpected and his friends were racked with pain. Among many acts of homage which were organised during the weeks following his death, there was a concert at the Musikverein, which included some works of Schubert and the first end of Don Giovanni. Among the performed lieder was the one we're hearing today, Aufenthalt, with a poem by Ludwig Rellstab and included in the Schwanengesang. The performers are Thomas Quasthoff and Justus Zeyen.
Wie sich die Welle
An Welle reiht,
Fließen die Tränen
Mir ewig erneut.
Hoch in den Kronen
Wogend sich's regt,
Mein Herze schlägt.
Und wie des Felsens
Bleibet mein Schmerz.