What can cause the flu or a cold?

The correct choice is that both a virus and bacteria can cause illnesses such as the flu or a cold. Specifically, the common cold is primarily caused by viral infections, with rhinoviruses being the most prevalent among them. The flu, or influenza, is also caused by viruses known as influenza viruses.

While bacteria are not the typical culprits for these respiratory illnesses—since they are mainly viral in nature—certain bacterial infections can sometimes develop as secondary infections following a viral cold or flu. For instance, bacteria can cause complications like sinusitis or pneumonia after an individual has already been affected by a viral infection.

Fungal infections play a much lesser role in respiratory illnesses like colds or flu and are typically associated with other conditions, such as fungal pneumonia or allergies. Hence, the main factors contributing to the flu and cold are indeed viral, supported by the occasional involvement of bacteria in secondary infections following the initial viral attack.