They do have bacteria on Mars, but, with far less variety or ferocity. Nevertheless, Maida’s aunt may be overstating things here.

↓ Transcript
Panel 1: Back at Maida's block of flats.
Panel 2: Maida's aunt is eating a bowl of curry at the kitchen table. Maida grabs some food to go. Ujana (in Swahili): "Why don't you sit down to eat?" Maida: "Can't. Studying. Exams start next week."
Panel 3: Ujana (in Swahili): "Don't leave food in your room or you'll catch a bacteria!" Maida (walking away): "Can't hear you! Finking about maffs!"
Panel 4: Maida closes the door to her room, a narrow space with clothes strewn about.
Panel 5: She lays on the bed and dips a steamed bun in her bowl of curry. Maida: "Alright, Papadum. Let's drill trigonometry next. Give me some nice, easy ones to start." Papadum: "Right-o."