A **disease** is any phenomenon or change that disrupts the normal function of the body, hence damaging homeostasis. There are various kinds of pathogens or *agents of disease* and the most common are **bacteria, fungi, worms, and viruses**. Other agents can be materials from an environment like cigarette smoke, a gene that is inherited (like the case of *hemophilia*).

A **disease** is any phenomenon or change that disrupts the normal function of the body, hence damaging homeostasis. A disease that is caused by a pathogen and is passed from one organism or medium to another is known as an ***infectious disease***.

Most of these infectious diseases are transmitted from one person to another through direct physical contacts like sneezing, coughing, or exposure to contaminated food materials or media. In rare cases, an infectious disease can also be transmitted from animal to person through direct exposure or vectors.

A **disease** is any phenomenon or change that disrupts the normal function of the body, hence damaging homeostasis. A disease that is caused by a pathogen and is passed from one organism or medium to another is known as an infectious disease.

Most of these infectious diseases are transmitted from one person to another through direct physical contacts like sneezing, coughing, or exposure to contaminated food materials or media. In rare cases, an infectious disease can also be transmitted from animal to person through direct exposure or vectors.

**Vectors** are animals or insects that carry pathogens from one person to another. In most cases, pathogens are carried through direct contact via *bites* or through indirect contact via their *secretions*. Some examples of vectors are the ***Anopheles*** mosquito that transmits dengue infection, **Sandflies** that causes Leishmaniasis, and the known **Tsetse fly** that carries the Trypanosoma infection.

**Antibiotics** are agents that kill or destroy bacterial species without harming the cells of the host. Antibiotics usually disrupt the cell membrane, or protein synthesis of the bacterial pathogen, thus damaging it. It is important to note that antibiotics only work with bacterial cells and not with viruses.

Koch required all four steps of his postulate in order to isolate the bacteria/virus that is causing the particular disease. All four steps are required, and **none** should be eliminated because the pathogen should be identified in order to know how to cure or eliminate it.

Koch’s postulates:

Step 1: The pathogen is found only in infected animals and **not in healthy ones**. If the pathogen was found in healthy animals, then his theory would automatically be proven wrong.

Step 2: The pathogen must be isolated and grown in pure culture. The pathogen must be isolated and re-grown in order to see if it is able to infect another animal

Step 3: The isolated/re-grown pathogen must infect a new host, causing the same disease as the original host. If the new host does not develop the same disease, then the pathogen is different from the original host.

Step 4: The pathogen must be re-isolated and be similar or the same as it is from the original host.