Airborne diseases refers to any diseases which are caused by pathogenic microbial agents and transmitted through the air. These viruses and bacteria can be aerosolized through coughing, sneezing, laughing or through close personal contact. These pathogens ride on either dust particles or small respiratory droplets and can stay suspended in air and or are capable of traveling distances on air currents.
Often, airborne pathogens or allergens cause inflammation

Inflammation

Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

 in the nose, throat, sinuses and the lungs. This is caused by the inhalation of these pathogens that affect your respiratory system or even the rest of your body.
Sinus congestion, coughing and sore throats are examples of inflammation of the upper respiratory air way due to these airborne agents.
Air pollution plays a significant role in airborne diseases which is linked to asthma

Asthma

Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...

. Pollutants are said to influence lung function by increasing air way inflammation. Alongside pollutants, tobacco

Tobacco

Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...

 smoke increases the risk of attracting these diseases.
Many common infections can spread by airborne transmission at least in some cases, including:

Anthrax

Anthrax

Anthrax is an acute disease caused by the bacterium Bacillus anthracis. Most forms of the disease are lethal, and it affects both humans and other animals...

 (inhalational), Chickenpox

Chickenpox

Chickenpox or chicken pox is a highly contagious illness caused by primary infection with varicella zoster virus . It usually starts with vesicular skin rash mainly on the body and head rather than at the periphery and becomes itchy, raw pockmarks, which mostly heal without scarring...

, Influenza

Influenza

Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...

, Measles

Measles

Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses...

, Smallpox

Smallpox

Smallpox was an infectious disease unique to humans, caused by either of two virus variants, Variola major and Variola minor. The disease is also known by the Latin names Variola or Variola vera, which is a derivative of the Latin varius, meaning "spotted", or varus, meaning "pimple"...

 and Tuberculosis

Tuberculosis

Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...

.
Airborne diseases are most commonly seen in unsanitary household conditions and overcrowded areas. Also, these diseases thrive in areas of poverty and poor hygienic conditions. Tuberculosis (TB) happens to be one of the most leading causes of death in adults from infectious diseases. Around 95 percent of people suffering from this disease are from developing areas in the world. Measles and diphtheria are two diseases found in poverty conditions, but over the years these diseases have been eliminated due to vaccines in developed countries.
Airborne diseases can also affect non-humans. For example, Newcastle disease

Newcastle disease

Newcastle disease is a contagious bird disease affecting many domestic and wild avian species. First found in Newcastle, United Kingdom in 1926, then by Burnet in 1943 in Australia in connection with laboratory infection where the virus was isolated from a ocular discharge of a patient to show the...

is an avian disease that effects many types of domestic poultry worldwide which is transmitted via airborne contamination.
Prevention

The best way to prevent attracting airborne diseases is to stay away from anyone who is affected, washing your hands regularly, and covering up your sneezes and coughs. This will reduce the risk of bacteria and pathogenic microbial agents entering your body through the air.
Symptoms

Often, airborne diseases can be as minor as the common cold or the typical flu. Symptoms would include: Coughing, sneezing, sinus congestion, itchy and watery eyes, sore throat, and fatigue. More serious diseases such as the ones listed above have more severe symptoms.