Communicable diseases (sometimes grouped with infectious diseases) are diseases that are contagious, which means they can be spread from one person or species to another. They can be caused by viruses, bacteria, protozoa, or fungi. When investigating communicable diseases, public health officials consider the following characteristics:
Mode of transmission – how a disease is spread. Examples include food, saliva, solid particles contaminating surfaces, inhalation, insect (vector), etc. depending on the disease type.
Ease of transmission – how easily the disease is spread from one host to another. This depends on several factors, including:
- How many organisms it takes to cause an infection
- How abundant or difficult to control the mode of transmission is (mosquitoes, for example, are abundant in some areas!)
- How resilient the organism is (can it survive on many kinds of surfaces in the open air for long periods of time, or does it need moisture to survive and be spread)
- Whether or not it can be carried in hosts without making them very sick (if it makes the host very sick, it may be less likely to spread because the host will not be able to move around enough to come into contact with many new potential hosts)
Incubation period – how long it takes for an infected person to develop symptoms from the time they come into contact with the pathogen. Diseases with longer incubation periods are more difficult to investigate and control, especially if it is easily transmitted.
Public health officials also consider other factors, such as how easy the disease is to diagnosis and treat, how long before and after symptoms arise does the host remain contagious, and many others. Communicable disease investigation is a tremendous resource in the public health protection ‘toolbox’.