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. Zoonotic diseases are infectious diseases of animals that can cause disease when transmitted to humans. Bacterial and viral infections have many things in common. Both types of infections are caused by microbes -- bacteria and viruses, respectively -- and spread by things such as: Coughing and sneezing, contact with infected people, Contact with contaminated surfaces, food, and water, Contact with infected creatures, including pets, livestock, and insects such as fleas and ticks. Microbes can also cause Acute infections, which are short-lived, Chronic infections, which can last for weeks, months, or a lifetime, Latent infections, which may not cause symptoms at first but can reactivate over a period of months and years, Most importantly, bacterial and viral infections, can cause mild, moderate, and severe diseases. Although bacteria and viruses are both too small to be seen without a microscope, they're as different as giraffes and goldfish. Bacteria are relatively complex, single-celled creatures, many with a rigid wall, and a thin, rubbery membrane surrounding the fluid inside the cell. They can reproduce on their own. Fossilized records show that bacteria have existed for about 3.5 billion years, and bacteria can survive in different environments, including extreme heat and cold, radioactive waste, and the human body. Most bacteria are harmless, and some actually help by digesting food, destroying disease-causing microbes, fighting cancer cells, and providing essential nutrients. Fewer than 1% of bacteria cause diseases in people.