You can smell other people’s mucous (not recommended). If you ever have been the primary care taker for a baby or small child you’d know what boogers smell like. Old boogers also start to decompose and the smell is faintly different. Sick mucous also smells different and worse. I could tell my nephews were sick with the flu by smelling them from ways away before noticing any other symptom. Sometimes I can smell that in public transport and it makes me figuratively run in the opposite direction of the sick person.