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.

Droplet Contact Also known as the respiratory route, it is a typical mode of transmission among many infectious agents. If an infected person coughs or sneezes on another person the microorganismsmay enter the body through the nose, mouth or eye surfaces. Diseases that are commonly spread by coughing or sneezing include (at least):

Bacterial Meningitis Chickenpox Common cold Influenza Mumps Strep throat Tuberculosis Measles Rubella Whooping cough

Direct contact is rare for this route, for humans at least. More common are the indirect routes  When food or water become contaminated  (by people not washing their hands before preparing food, or untreated sewage being released into a drinking water supply) and the people who eat and drink them become infected.  Happens in less developed countries Examples of Oral Transmission Cholera Hepatitis A Polio Rotavirus Salmonella

This refers to any disease that can be caught during sexual activity with another person, including vaginal or anal sex or (less commonly) through oral sex. Transmission is either directly between surfaces in contact during intercourse (the usual route for bacterial infections and those infections causing sores) or from secretions (semen or the fluid secreted by a female) which carry infectious agents that get into the partner's blood stream through tiny tears in the penis, vagina or rectum (this is a more usual route for viruses). Some diseases transmissible by the sexual route include: HIV/AIDS Chlamydia Genital warts Gonorrhea Hepatitis B Syphilis Herpes

Diseases that can be transmitted by direct contact are called contagious. These diseases can also be transmitted by sharing a towel (where the towel is rubbed vigorously on both bodies) or items of clothing in close contact with the body (socks, for example) if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms. Some diseases that are transmissible by direct contact include: Athlete's foot Impetigo Syphilis (on rare occasions, if an uninfected person touches a chancre) Warts

A vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another. Examples include mosquitoes, deer ticks, animal bites, etc.

Non-infectious diseases (also called Non- communicable diseases ) are those diseases that are not caused by a pathogen and cannot be shared from one person to another. Non-infectious diseases may be caused by either the environment, nutritional deficiencies, lifestyle choices, or genetic inheritances. Unlike infectious diseases, non- infectious diseases are not communicable or contagious, although some kinds can be passed down genetically to the children of a carrier.

Historically, infectious diseases were the main cause of death in the world and, indeed, in some developing regions this may still be the case. With the development of antibiotics and vaccination programs, infectious disease is no longer the leading cause of death in the western world. Non-infectious disease is now responsible for the leading causes of death in both developed and some developing countries.

Causes of non-infectious disease are categorized into genetically inherited diseases and environmental diseases. Non-infectious diseases that are not genetic disorders are environmental diseases, although many diseases are affected by both genetic and environmental factors.

Environmental disease is a very broad category. It includes avoidable and unavoidable conditions caused by external factors, such as sunlight, food, pollution, and lifestyle choices. The diseases of affluence are non-infectious diseases with environmental causes. Examples include: Many types of cardiovascular disease Chronic obstructive pulmonary disease caused by smoking tobacco Type 2 Diabetes Malnutrition caused by too little food, or eating the wrong kinds of food (e.g. scurvy from lack of Vitamin C) Skin cancer caused by radiation from the sun