Most of the elements present in the environment, such as chemicals, pollutants, pathogens, and toxins, make their way into the human body and cause diseases. The body’s natural defense mechanism is the immune system, and whenever a foreign body finds its way into the bloodstream, this system immediately acts up to get rid of it. In many cases, however, the immune system is unable to completely battle the infections that make their way into the body.

Infectious diseases are among the most common cause of death worldwide, forty years after the introduction of antibiotics.

Infectious diseases involve much more than growth of microbes or parasitic animals in the body. The factors that determine the initiation, development, and outcome of an infection involve a series of complex and shifting interactions between the invading organism and the host which can vary with different infecting organisms. These interactions include the following:

• The organism’s ability to breach host barriers and to evade destruction by innate local and tissue host defenses.
• The organism’s biochemical tactics to replicate, to establish infection, and to cause disease.
• The microbe’s ability to transmit to a new susceptible host.
• The body’s innate and adaptive immunologic ability to control and eliminate the invading parasite.

Infectious diseases are the second leading cause of death worldwide and the leading killer of infants and children, however. In the United States, infectious diseases are the third leading cause of death and infectious disease mortality increased during the last twenty years of the century. In addition, over the past twenty five years there have been numerous reminders regarding the challenges infectious diseases will continue to pose domestically and globally.

Further more, when a human population comes in contact with a new transmissible agent, or with a previously isolated human population, new infectious agents are likely to enter the human community. The infectious agents accounted for the spread of measles, smallpox, influenza, and bubonic plague in the centuries past.

The increase in the occurrence of infectious problems has led to the identification of rising infectious diseases; these have been explained in abundant reports. It is relevant to note that ten to fifteen years ago it was thought that most infectious agents of diseases had been identified and that the few agents still unknown played a role in diseases, like cancer, which manifested them many years after infection.

Immunizations can prevent from death from infectious diseases for individuals and can help control the spread of infections. The objectives selected to measure progress in this area are:

• Increase the proportion of young children who receive all vaccines that have been recommended for universal administration for at least five years.
• Increase the proportion of non-institutionalized adults who are vaccinated annually against influenza and ever vaccinated against pneumococcal disease.

The defense mechanisms of the body are complex and include innate mechanisms and acquired systems. Innate or non-specific immunity is present from birth and includes physical barriers, chemical barriers, phagocyte cells and the complement system.

The common cold, also called acute coryza, is caused by coronaviruses or picornaviruses. It is highly infectious and can be transmitted through the air, or from contact with contaminated items or dirty hands of an infected person, especially when one touches his nose or eyes. Symptoms of this condition include sneezing, mucus secretion from the nose, and some coughing. It is recommended to always sanitize the hands to prevent the spread of the virus. Also, supplementing Vitamin C in the diet helps increase immunity against the common cold.

Another infectious disease that affects the respiratory system is pneumonia, which is mostly confined to the lower respiratory tract. The species of bacteria that commonly causes pneumonia is Streptococcus pneumonia, which is a spherical shaped gram positive bacterium. However, pneumonia may also be caused by viruses or fungi. The alveoli of the lungs are filled with dead white blood cells and fluid, preventing effective gas exchange and causing the lack of oxygen in the body. The flooding of the lungs with fluid may also be caused by injury to the lungs. Pneumonia can be treated with antibiotics, and there are currently vaccines that help prevent it.

Tuberculosis (TB) is another infectious disease that is brought about by bacteria, specifically Mycobacterium tuberculosis, which is also a gram positive species. The bacteria find their way into the lungs through breathing, or via the bloodstream and lymphatic system. When the bacteria have incubated in the lung tissue, they decrease the elasticity of the lungs. A patient with TB may experience chills, fevers, fatigue, weight and appetite loss, as well as coughing with blood-stained sputum. The antibiotic drugs used to treat TB are isoniazid and rifampicin.

There are also infectious diseases that are not confined to the respiratory system, such as Acquired Immunodeficiency Syndrome (AIDS) or Human Immunodeficiency Virus (HIV), which are generally transmitted via the blood. AIDS and HIV are among the deadliest diseases in the world today, with still no known cure or treatment. Pregnant mothers who are diagnosed with HIV or AIDS also put their unborn babies at risk of acquiring the disease. These viral infections compromise the normal functions of the immune system, preventing effective defences against other diseases.