For instance, the Bible does mention how to deal with issues of bioethics, modern financial and economic systems, and many other topics because they simply didn't exist at the time.
So we need Church authorities to apply and interpret these teachings to modern issues. The Pope can and should and does tell us how to apply Catholic teaching in modern situations, but he might communicate poorly (this is a real problem with Pope Francis), or even make mistakes.
Papal infallibility is very specifically and narrowly defined and is rarely applicable. It has only been invoked twice in the last 150 years, both times for doctrinal statements relating to Mariology (i.e., Mary the Mother of God, not the monkey-fighting plumber guy, although I find that idea amusing).
In both cases, Pope Pius IX and Pius XII were establishing as doctrine teachings that had been believed and taught for centuries, and which refined, but did not fundamentally change anything.
Popes sometimes refer to an infallible statement, but this is merely a reiteration of existing doctrine (e.g., When Pope John-Paul II invoked infallibility in the restatement that the Church does not have the authority to ordain women, but he wasn't saying anything new).
The Pope's authority is valid only in as much as he is consistent with the teachings passed down from the Apostles, and when he is making statements that are consistent with Catholic teaching and in communion with the bishops and traditions of the Church.
As an example, Pope Francis' claims that capital punishment is always wrong contradicts traditional Catholic teaching. Now the application of capital punishment in a modern society may be unacceptable for reasons of the protection of rights of the accused (e.g., overturning a wrongful conviction doesn't any good if the no-longer-a-convict is dead), maintaining safety (e.g., life imprisonment keeps society safe from people who are too dangerous to be in society), and the process is just too unwieldy to be exercised in an efficient and just way (e.g., people being on death row for decades through endless appeals).
But this does not mean that capital punishment is inherently wrong, and this is what the Church has always taught. Francis is expressing an opinion here, one that is worth considering, but he is not changing Church teaching because he can't.
For the record, I don't support capital punishment, but for reasons stated above, not because it's inherently wrong, but because it is not necessary. If I were living in a small community on an isolated island, or in many other situations, my opinion might be different.