How Are Diseases Transmitted?



Infectious diseases are transmitted from person to person by direct or indirect contact. Certain types of viruses, bacteria, parasites, and fungi can all cause infectious disease. Malaria, measles, and respiratory illnesses are examples of infectious diseases. 

Simple preventative measures, such as frequent hand washing, can cut down on disease transmission.  

Direct contact 

Infectious diseases are often spread through direct contact. Types of direct contact include:  

1. Person-to-person contact 

Infectious diseases are commonly transmitted through direct person-to-person contact. Transmission occurs when an infected person touches or exchanges body fluids with someone else. This can happen before an infected person is aware of the illness. Sexually transmitted diseases (STDs) can be transmitted this way. 

Pregnant women can also transmit infectious diseases to their unborn children via the placenta. Some STDs, including gonorrhea, can be passed from mother to baby during childbirth. 

2. Droplet spread 

The spray of droplets during coughing and sneezing can spread an infectious disease. You can even infect another person through droplets created when you speak. Since droplets fall to the ground within a few feet, this type of transmission requires close proximity. 

Indirect contact 

Infectious diseases can also be spread indirectly through the air and other mechanisms. For example:  

1. Airborne transmission 

Some infectious agents can travel long distances and remain suspended in the air for an extended period of time. You can catch a disease like measles by entering a room after someone with measles has departed. 

2. Contaminated objects 

Some organisms can live on objects for a short time. If you touch an object, such as a doorknob, soon after an infected person, you might be exposed to infection. Transmission occurs when you touch your mouth, nose, or eyes before thoroughly washing your hands. 

Germs can also be spread through contaminated blood products and medical supplies. 

3. Food and drinking water 

Infectious diseases can be transmitted via contaminated food and water. E. coli is often transmitted through improperly handled produce or undercooked meat. Improperly canned foods can create an environment ripe for Clostridium botulinum, which can lead to botulism.  

4. Animal-to-person contact 

Some infectious diseases can be transmitted from an animal to a person. This can happen when an infected animal bites or scratches you or when you handle animal waste. The Toxoplasma gondii parasite can be found in cat feces. Pregnant women and people with compromised immune systems should take extra care (disposable gloves and good hand washing) when changing cat litter, or avoid it altogether. 

5. Animal reservoirs 

Animal-to-animal disease transmission can sometimes transfer to humans. Zoonosis occurs when diseases are transferred from animals to people. Zoonotic diseases include:  

anthrax (from sheep) 

rabies (from rodents and other mammals) 

West Nile virus (from birds)  

plague (from rodents) 

6. Insect bites (vector-borne disease) 

Some zoonotic infectious agents are transmitted by insects, especially those that suck blood. These include mosquitos, fleas, and ticks. The insects become infected when they feed on infected hosts, such as birds, animals, and humans. The disease is then transmitted when the insect bites a new host. Malaria, West Nile virus, and Lyme disease are all spread this way. 

7. Environmental reservoirs 

Soil, water, and vegetation containing infectious organisms can also be transferred to people. Hookworm, for example, is transmitted through contaminated soil. Legionnaires’ disease is an example of a disease that can be spread by water that supplies cooling towers and evaporative condensers. 

Media Contact:  

Allison Grey  

Journal Manager  

Journal of Infectious Diseases and Diagnosis