The most dangerous things are often the tiniest, the smallest, the things we can not see. This statement is debatable but with regard to infectious diseases, is a clear winner. For infections are caused and to a large extent, spread by tiny organisms, invisible to the naked eye and causing havoc within the body. These diminutive villains are none other than pathogens, which are different species of viruses, bacteria fungi and some parasites. They infect, destroy, multiply and distribute themselves. Infectious diseases are characterized by a foreign agent or body and spreadable or contagious nature. While any body part or organ is vulnerable to the dangerous effects of such microorganisms, certain areas are more delicate and an infection can result in life-threatening complications. One such area is the human respiratory system, which includes the lungs, the bronchi and the breathing mechanism muscles and nerves. Diseases which affect the respiratory system are truly unsafe, as their contagious nature and the important body area they affect, make for a dangerous combination. Infectious Diseases That Affect the Respiratory System Below, some of the contagious diseases, that affect or originate in the respiratory organs are listed, with a description of the important medical points: ✠ Diphtheria

Feature	Description
Infecting Agent	Corynebacterium diphtheriae (Bacteria)
Site of Infection	Upper respiratory tract illness, nose and throat
Contagion Mechanism	Any physical contact with infected person, through air, food or objects
Key Symptoms	Difficulty breathing, throat infection, fever, mucus and thick cough, neck area is swollen or enlarged
Complications	Heart problems (mycarditis), kidney malfunctioning, nerve paralysis, permanent breathing problems. 10% of infected cases could die
Preventive Measures	DTaP vaccine and booster shots

✠ Pertussis (Whooping Cough)

Feature	Description
Infecting Agent	Bordetella pertussis (Bacteria)
Site of Infection	Upper respiratory infection
Contagion Mechanism	Sneezing allows the bacteria to enter the air, where it can spread to nearby individuals
Key Symptoms	Deep and continuous hacking cough, runny nose, vomiting after coughing, mild fever
Complications	Earaches, pneumonia and seizures. Infants (under 6 months) and toddlers are at risk of such complications. Infants are also at risk of fatalities. Adults and older children can recover completely
Preventive Measures	DTaP vaccine for infants and young children. For older children and adults, Tdap vaccine should be given

✠ Pneumonia

Feature	Description
Infecting Agent	Multiple agents, either bacteria, fungi, parasites or viruses
Site of Infection	Lungs
Contagion Mechanism	Breathing in germs, which spread through the air, from an infected person
Key Symptoms	Fever, feeling tired with no appetite, sweating and shaking, coughing with a stabbing chest pain, breathing difficulty
Complications	Abscesses are formed in the lungs(Empyema). Severe and long term failure of the respiratory system. Sepsis and ARDS can also occur
Preventive Measures	Seasonal flu shot and pneumococcal vaccine help in protection

✠ Tuberculosis

Feature	Description
Infecting Agent	Mycobacterium tuberculosis (Bacteria)
Site of Infection	Lungs
Contagion Mechanism	When a TB patient sneezes, spits or coughs, anyone around him/her can get infected from the germs present in the air
Key Symptoms	Prolonged coughing, sometimes blood is coughed up, sharp chest pain, loss of appetite and pale skin tone, fever with chills and sweating
Complications	Permanent lung damage can occur. TB if allowed to worsen, will cause death
Preventive Measures	The BCG vaccine is used in most countries, but is not completely risk-proof

✠ Primary Pulmonary Histoplasmosis

Feature	Description
Infecting Agent	Histoplasma capsulatum (Fungus)
Site of Infection	Lungs
Contagion Mechanism	The fungus is present in bird and bat droppings and soil. Inhalation of the spores will cause the disease, it does not spread from person to person
Key Symptoms	Chest discomfort, aches and pains in joints and muscles, mild fever and cough, rash. Most cases do not show symptoms
Complications	The infection can spread to other organs, if the immune system is weak. If allowed to worsen, the condition can become chronic or permanent
Preventive Measures	No vaccine exists. At best, endemic areas with contaminated soil should be avoided. These include the Mississippi and Ohio River Valleys

✠ Acute Bronchitis

Feature	Description
Infecting Agent	Viral (rhinovirus) or bacterial pathogens
Site of Infection	The bronchi or airways of the lungs
Contagion Mechanism	Spreads through physical contact with infected person. Can also spread through air
Key Symptoms	Coughing with mucus production, mild fever, wheezing and fatigue
Complications	Repeated occurrences leads to chronic bronchitis. Asthma, tuberculosis and sinusitis are long-term conditions, if condition worsens
Preventive Measures	Annual influenza vaccine helps in prevention. A pneumonccocal vaccine reduces the risk of pneumonia

It's clear from the above listing of diseases of the respiratory system, that such conditions have very serious consequences and can cause fatalities. Certain diseases like diphtheria have nearly been eradicated in the U.S. But in third-world countries, lack of vaccination drives and unsanitary living conditions can result in an infection and a rapid rate of contagion, ultimately resulting in an epidemic. The best defense against infectious diseases of the respiratory system or any other body part is prevention, through on-time and effective vaccinations or quick quarantine and treatment of infected cases, to prevent it from spreading.