Which of the following is not classified as an infectious disease?

Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid in the brain's ventricles, leading to increased pressure within the skull. It is generally caused by congenital defects, brain tumors, infections, or trauma, but it is not classified as an infectious disease itself. In contrast, pneumonia, HIV/AIDS, and tuberculosis are all classified as infectious diseases because they are caused by pathogens like bacteria and viruses that can be transmitted between individuals.

Pneumonia, for instance, typically results from infections caused by bacteria or viruses, which can lead to inflammation in the lungs. HIV/AIDS is caused by the human immunodeficiency virus, which attacks the immune system. Similarly, tuberculosis is caused by Mycobacterium tuberculosis, a bacterium that primarily affects the lungs but can also impact other parts of the body. Each of these conditions represents examples of how infectious agents can spread and cause disease, while hydrocephalus is primarily a pathological condition related to fluid dynamics within the brain rather than an infectious process.