In this week's Parsha, we read that Yosef is told "הנה אביך חולה," behold, your father is sick (בראשית מ"ח:א'). The Gemara tells us in Bava Metzia (דף פ"ז.) that until the time of Yaakov, there was no illness in the world. Yaakov, however, wanted illness in the world because he knew that people will do Teshuvah when faced with troubles. If one would become ill and realize that his life was in danger, he might decide to do Teshuvah. Our goal in life should be to repent and mend our ways. The maximum life span of a human after Moshe, according to our tradition, is one hundred and twenty years, during which one should repent. It is interesting to note that Noach spent one hundred and twenty years building the ark, in order to give the other people of his day time to repent. We thus see that the number one hundred and twenty is somehow associated with Teshuvah.
This statement, that there was no illness in the world until Yaakov, apparently contradicts a statement of the Gemara in Sotah (דף י"ד.), which teaches that Hashem appeared to Avraham in order to visit the sick (עיין בראשית י"ח:א וברש"י שם). Since this is quoted as a source for the act of Chessed called Bikur Cholim, visiting the sick, we see from here that there was sickness before Yaakov.
The Gemara in Bava Basra (דף ט"ז:) moreover, says that Avraham Avinu had a precious pendant hanging around his neck which healed any sick person who gazed at it. As a side point, Chazal teach us that when one comes into contact with a Tzaddik, some of his holiness rubbes off; it was thus not the pendant that healed the sick, but rather Avraham. In any case, we see from here too that there was sickness before Yaakov.
Tosafos in Bava Basra (שם בד"ה שכל) raises our question and answers that there was sickness, but before Yaakov, the sickness did not cause death. Unless a person is faced with possible death, he usually will not repent; Yaakov thus prayed for the kind of sickness which causes death. Some of these thoughts are based on the Sefer Iturei Torah.