Explanation:

- Malaria: A disease caused by a parasite, transmitted by the bite of infected mosquitoes.

- Flu: Short for influenza, a contagious respiratory illness caused by viruses.

- Typhoid: A bacterial infection due to Salmonella typhi, leading to high fever and other symptoms.

- Depression: A mental health disorder characterized by persistently depressed mood and loss of interest.

The first three options — Malaria, Flu, and Typhoid — are diseases caused by pathogens (parasites, viruses, bacteria). Depression is distinct as it is a mental health condition, not caused by a pathogen. Therefore, the odd one out is:

- Depression