Which of the following is a cause of infectious disease?

The cause of infectious diseases is primarily linked to pathogens, which include microorganisms such as bacteria, viruses, parasites, and fungi. Fungi can lead to a range of infections, particularly in individuals with weakened immune systems. For example, fungal infections like candidiasis or aspergillosis can seriously affect those who are immunocompromised, showing clear evidence of how fungi can cause infectious diseases.

In contrast, genetic illnesses are caused by mutations or abnormalities in genes and are not infectious. Blood sugar levels and cholesterol levels reflect metabolic or physiological conditions but do not directly lead to infections caused by pathogens. Thus, fungi are recognized as a significant cause of infectious disease, making them the correct answer in this context.