My teacher, Ernesto Pichardo--Obá Irawó, likes rhetorical questions, so one day he asked me, "What odu does Babalú-Ayé appear in?" I mentioned that people say that Babalú is born in the sign Odí-Eyeunle, along with vomit and smallpox. He said, "Yes, that is true, but there is sickness in every sign, and so Babalú is in every sign. In this he is like Elegguá, who appears everywhere." It is true. The sign Oché Meyi speaks of problems with the blood and diseases like leukemia. The sign Iroso-Ofún speaks of impotence. In the treatises that compile the wisdom about the signs, each one speaks to particular diseases or vectors of infection.
I have heard that some Yoruba babalawos always mark an offering for Eshu, and then one for Babalú-Ayé, who has immense power. "Always" is probably a figure of speech, but it does point to a pattern: Babalú-Ayé is offered something in every odu.
Babalú-Ayé is strongly associated with the Earth itself, and West Africans and their descendants in Diaspora fondly point out that we always stand on the Earth. Anthropologist Melville Herskovits found that Fon people in present-day Benin make oaths upon the Earth, precisely because it is everywhere and witnesses everything. To seal an oath, they make a concotion with water and crushed herbs, and then they add Earth to it. The parties to the oath then drink this concoction, invoking the Earth as the true witness to their commitment.
When Lucumí people make a Babalú-Ayé osain, a concoction made with white wine, water from coconuts, and herbs used to purify and empower the oricha, they too add Earth to the mix. (Osains usually take water, but Babalú has a deep aversion to water and so takes other liquids.) Armando Zulueta's lineage adds Earth from where the oricha was buried or from the inside of the oricha's pot. There are Matanzas lineages that take Babalú-Ayé and feed him at the foot of a ceiba tree and in a cemetary; they feed him a guinea hen and then pick up Earth, which they later add to the osain.
Appearing in every divination sign--every imaginable situation--and linked inexorably with the Earth itself, Babalú-Ayé is ubiquitous and witness to all of our actions.