Infectious Diseases

Sources and strategies for treatment

Infectious diseases are disease states resulting from the body being invaded by a foreign agent — most typically a microorganism — that impairs the health of an infected individual. In many (but not all) cases, infectious diseases are also communicable, and can be directly or indirectly spread from person to person.

Although there are various ways of categorizing infectious diseases depending on the context (such as by geographic region or degree of communicability), one of the most common methods of classification is by the type of causative agent. Infectious diseases can result from bodily invasion by several different types of organism.

• Bacterial infections – Most commonly caused by strains of streptococci, staphylococci and pneumococci; chief concern is unchecked proliferation leading to septicemia (bacteria entering the bloodstream) or even sepsis: body-wide inflammation resulting in tissue damage or clotting that can cause organ failure. These infections can be treated with antibiotics that inhibit bacterial growth and/or replication.
• Viral infections – Caused by viruses that can only replicate (and cause disease) by redirecting host-cell machinery to make and assemble viral components. Viruses can emerge or re-emerge into human populations by “jumping” from animal reservoirs through interspecies contact (as in the cases of SARS coronavirus, influenza A H1N1, human immunodeficiency virus (HIV/AIDS), and SARS-CoV-2 coronavirus. Viruses are not susceptible to antibiotic treatment.
• Fungal infections – Also known as mycoses; causative fungi can exist and infect as unicellular organisms (yeasts) or multicellular, filamentous structures (molds). Related diseases include histoplasmosis, coccidioidomycosis and blastomycosis, which can be life-threatening to immunocompromised individuals. These infections are often treatable with specific antibiotics called antifungals.
• Parasitic infections – Can be caused by proliferation of microorganisms (such as malaria-causing protozoans) or even a single, much larger organism such as the tapeworm. The infectious lifecycle of many parasitic worms can involve the GI tract, bloodstream and/or other body systems. These infections can be debilitating and even fatal, but are usually readily diagnosable and treatable with drugs.

The World Health Organization (WHO), along with other international public health entities and national governments on all levels have historically prioritized controlling the spread of infectious diseases through campaigns and guidance on vaccination, screening, quarantining and continuing research.