Infectious diseases are disorders caused by organisms-such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one person to another. Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response. While the most common infectious diseases are viruses that cause mild short-term effects, other infectious diseases have the potential to cause longer term or more serious effects.