Transmission is the act or process of spreading a disease. Diseases occur when a person becomes infected with a disease-causing microbe called a pathogen. There are a variety of different types of pathogens, including:

• Bacteria
• Viruses
• Parasites, including protozoa (single-celled organisms) and helminths (multicellular worms)
• Fungi

When a pathogen enters the body, it multiplies. Sometimes the immune system is able to fight off the infection. But other times, the pathogen multiplies too quickly for the immune system to eliminate it. This results in disease.

Diseases can be spread in a variety of ways, including:

• From person to person. This is the most common method by which disease is spread. It can involve respiratory, sexual, mother-to-child (vertical) or surface contact (fomite) transmission.
  
  
• From an animal or insect to a person. Diseases are often spread from animals and insects to people through a bite. Infected animals and insects may harbor pathogens in their saliva, which are then transmitted to people through a bite if the saliva enters the bloodstream. Animals or insects that are responsible for transmitting diseases are called vectors.
  
  
• From eating or drinking contaminated food or water. In the United States, foodborne and waterborne diseases are not as common as in some developing countries. However, some pathogens can be carried in food or water. The salmonella bacteria, which cause salmonellosis, are among the most common foodborne bacteria.
  
  
• From a blood transfusion or organ transplant. Since the 1980s, blood and organ tissue has been routinely screened for the presence of certain diseases such as hepatitis, HIV and syphilis. This has significantly reduced the instances of transmission of those diseases from blood transfusions or organ transplants. There are, however, rare cases of other diseases such as West Nile virus and cytomegalovirus being transmitted in this way.

It is impossible to completely avoid pathogens, but there are steps that people can take to reduce their risk of infection. These include practicing good personal hygiene, ensuring sanitary food preparation, maintaining a strong immune system and keeping vaccinations up to date.