What is an infectious disease? An infectious disease is a type of illness that is caused by microorganisms such as bacteria, viruses, fungi, or parasites. These microorganisms can enter the body and multiply, leading to an infection. Infectious diseases can be spread from person to person through various means, including direct contact, respiratory droplets, contaminated food or water, or through vectors such as mosquitoes or ticks. Some examples of infectious diseases include the common cold, influenza, tuberculosis, malaria, and HIV/AIDS. It is important to note that not all infections are considered infectious diseases, as some infections may be caused by non-infectious factors such as allergies or autoimmune disorders.