Introduction:
Communicable diseases, also known as infectious diseases, are illnesses caused by microorganisms such as bacteria, viruses, parasites, or fungi that can be spread from one person to another or from animals to humans. These diseases can be transmitted through direct contact, airborne droplets, contaminated food or water, or through vectors such as mosquitoes or ticks.

Explanation:
Communicable diseases are categorized as infectious diseases because they are caused by infectious agents that can invade the body and multiply, leading to illness. The term "communicable" emphasizes that these diseases can be transmitted or spread from one individual to another.

Types of diseases:
There are two main types of diseases: communicable (infectious) and non-communicable (non-infectious). However, in the case of communicable diseases, they are also considered infectious diseases. Hence, the correct answer is option 'A' - infectious.

Communicable diseases:
Communicable diseases include a wide range of illnesses such as:

1. Respiratory infections: These include diseases like the common cold, influenza, tuberculosis, and pneumonia, which can be spread through respiratory droplets when an infected person coughs or sneezes.

2. Sexually transmitted infections (STIs): Diseases like HIV/AIDS, syphilis, gonorrhea, and chlamydia can be transmitted through sexual contact with an infected person.

3. Gastrointestinal infections: Illnesses like food poisoning, cholera, hepatitis A, and rotavirus infection can be contracted through contaminated food or water.

4. Vector-borne diseases: These diseases are transmitted by vectors like mosquitoes, ticks, or fleas. Examples include malaria, dengue fever, Zika virus, and Lyme disease.

5. Bloodborne infections: Diseases like hepatitis B and C, HIV/AIDS, and Ebola can be spread through contact with infected blood or body fluids.

6. Skin and mucous membrane infections: Diseases like impetigo, ringworm, scabies, and sexually transmitted infections like genital herpes can be transmitted through direct contact with infected skin or mucous membranes.

Conclusion:
Communicable diseases, also known as infectious diseases, are caused by microorganisms and can be transmitted from one person to another. These diseases encompass a wide range of illnesses and can be spread through various modes of transmission. It is important to practice good hygiene, follow preventive measures, and seek medical attention when necessary to prevent the spread of communicable diseases.

A