Now that you know something about what communicable diseases are, in this activity, you are going to learn about how they are spread.

Communicable diseases are transmitted in five ways:

• through the air

•  through water

• through direct contact

• through indirect contact

• through a carrier that can transmit a disease from one species to another

Some of these methods of transmission may seem quite obvious, but others might be a little harder to understand. As well, some diseases can be spread in several of these five ways.

Diseases that are transmitted through the air include colds, the flu, polio, tuberculosis, and whooping cough. Coughs and sneezes carry the virus in invisible droplets that spread through the air and are breathed in by someone else, allowing the virus to reproduce and make that person sick as well.

Diseases that are spread through water are very common in places where water treatment standards are not very high or even nonexistent. Bacteria and parasites reproduce in water where no cleaning agents such as chlorine are present. People who drink or wash with that water can then become sick. Some very common waterborne illnesses are giardiasis, sometimes called "beaver fever," which causes diarrhea, cholera, typhoid fever, and Hepatitis A and E.

Many diseases are spread through direct contact with an infected person. For example, some kinds of warts can be contagious when you come in contact with them. Sexually transmitted diseases such as syphilis and chlamydia are transmitted through direct sexual contact. Ringworm and lice are transmitted through direct contact with an infected person's skin or hair.

Some diseases are spread through indirect contact. That is, they are spread when a person comes in contact with something that a sick person has been in contact with as well. For example, some organisms such as the Norwalk virus can live for a long time on surfaces such as tables, telephones, bedding, or children's toys.

Some diseases are also spread by a carrier —  or what scientists call a "vector"  — that can transmit a disease from one species to another. Lyme disease is transmitted from deer to humans through ticks. Malaria is transmitted in this way when certain mosquitoes transmit it in their saliva when they withdraw blood from one person and then another.

Click on the + sign if you would like to look at this image more closely.

 Now that you know about some of the ways in which communicable diseases can be transmitted, the next activity will explore ways that you can prevent the spread of diseases.