Infectious Diseases: A Guide to Common Illnesses, Their Causes, Risks, and Treatments

Infectious diseases are illnesses caused by pathogens such as bacteria, viruses, fungi, or parasites. These disease-causing microorganisms can spread from person to person, animal to person, or via insect vectors like mosquitoes. Infectious diseases remain a major global health challenge, but medical advances have enabled us to better prevent, diagnose, and treat many common infections.

What is the Difference Between Disease and Illness?

Before diving into specific illnesses, it helps to understand the difference between a disease and an illness:

  • Disease refers to a specific condition with an identified cause and recognized signs and symptoms. Diseases have a defined medical diagnosis and prognosis.
  • Illness is a broader term referring to a person feeling unwell due to any cause, whether diagnosed or not. An illness may or may not have an identified disease causing it.

So in short, a disease is the actual medically explained condition, while illness describes the subjective feeling of being unwell. Infectious diseases lead to illness when the infection makes someone feel sick.

Most Common Infectious Diseases

The World Health Organization (WHO) tracks data on incidence rates around the world to determine the most widespread infectious diseases. The following are among the most common globally:

  • Influenza – Caused by influenza viruses, the flu leads to about 3 to 5 million severe illnesses and 290,000 to 650,000 deaths annually worldwide. Influenza spreads rapidly via respiratory droplets when infected people cough, sneeze, or talk.
  • Tuberculosis – An estimated 10 million people fell ill with tuberculosis (TB) in 2021. TB spreads through the air and primarily affects the lungs, causing coughing, chest pain, fatigue, fever, and unintentional weight loss.
  • Malaria – About 241 million malaria cases and 627,000 related deaths occur worldwide each year, mostly among young children in sub-Saharan Africa. The parasite spreads to people through mosquito bites.
  • Hepatitis B – Approximately 296 million people globally live with chronic hepatitis B virus infection, which can cause liver inflammation, cirrhosis, liver failure, and liver cancer. It spreads via contact with blood or other body fluids.
  • HIV – An estimated 38.4 million people have HIV worldwide. The virus attacks the body’s immune system and spreads via sexual contact, shared needles, blood products, and from mother to baby during pregnancy and delivery.
  • Dengue – More than 50-100 million dengue virus infections happen every year around the tropics and subtropics. Spread by Aedes mosquitoes, it causes flu-like illness that can develop into severe dengue or dengue hemorrhagic fever.

Beyond this list, lower respiratory infections, and diarrheal diseases like rotavirus, and meningitis also cause considerable infectious disease burden, along with endemic diseases like typhoid and leprosy in some countries.

Other Common Human Diseases and Conditions

In addition to infectious illnesses, various non-communicable diseases and chronic conditions affect people around the world. A 2021 analysis found the most common health conditions globally to be:

  1. Dental caries (tooth decay)
  2. Tension headaches
  3. Iron deficiency anemia
  4. Loss of eyesight
  5. Hearing loss
  6. Skin disorders like dermatitis
  7. Migraines
  8. Anxiety disorders
  9. Neck pain
  10. Other musculoskeletal disorders like lower back pain

This list highlights how oral, mental, skin, sensory, and musculoskeletal conditions widely impact well-being and quality of life along with infectious diseases. However, their non-contagious nature means different public health approaches are required.

Illness vs. Disease: Key Differences

As mentioned, illness refers to feeling sick or unwell, which can occur due to:

  • Diagnosed disease – Such as having nausea, fatigue, and dehydration due to confirmed norovirus infection
  • Undiagnosed disease – For instance, experiencing abdominal cramping and loose stools from what may be viral gastroenteritis before getting tested
  • Non-disease causes – Such as headaches, body aches, and nausea from stress rather than a specific physical illness

Alternatively, disease refers specifically to any condition with an identified, medically explained cause including:

  • Bacterial, viral, fungal, or parasitic infectious diseases – Such as COVID-19 (coronavirus disease)
  • Chronic diseases – Like diabetes, asthma, heart disease, and autoimmune conditions
  • Genetic diseases – Such as cystic fibrosis, sickle cell anemia, and Huntington’s disease
  • Nutritional deficiency diseases – Including iron deficiency anemia, scurvy from vitamin C deficiency, and bone conditions from low calcium, vitamin D, or phosphate
  • Degenerative diseases – Such as Alzheimer’s, Parkinson’s, arthritis, vision loss, and many forms of kidney disease

Thus illness is a subjective state that may or may not have an identified disease process behind it. Identifying any underlying disease can clarify the prognosis and guide appropriate treatment.

How Infection Causes Illness

Infectious pathogens can invade the body and multiply, causing cellular damage while resisting the immune system. This microbial invasion triggers an immune and inflammatory response to fight the infection. But the resulting cellular battle and chemicals released make people feel ill by causing symptoms like:

  • Fever – Higher body temperature from pyrogens released by immune cells
  • Fatigue & malaise – Generalized exhaustion as the body shunts energy towards combatting infection
  • Congestion & discharge – Swelling, fluids, and immune cells rush to infected tissue sites
  • Pain – From inflammation and/or tissue damage
  • Change in taste/smell – Chemoreceptors altered by congestion and neurological impact
  • Nausea & vomiting – Activates from neuro-immune interactions and systemic inflammation
  • Diarrhea – Intestinal osmolarity disrupted, gut motility altered

In vulnerable groups like infants, the elderly, and immunocompromised individuals, infectious illnesses can also turn deadly if the unchecked infection spreads systemically or cytokines trigger severe inflammatory responses.

However, in healthy individuals, short-term symptomatic illness is a normal and generally protective immune response to clear microbes and develop lasting immunity without serious or lasting issues. However chronic diseases emerge if pathogens or inflammation persist.

Reducing Infectious Disease Risk

While some exposure to infectious microbes trains our immune systems, unnecessary infection risks that make large swaths of the population ill are concerning from both public health and socioeconomic perspectives.

We can reduce infection exposure risks through measures like:

  • Sanitation infrastructure – Safely managing human waste, water systems, food handling, etc
  • Routine vaccination – Following recommended schedules to develop immune memory against vaccine-preventable diseases
  • Vector control – Limiting bites from parasite-carrying insects via protective clothing, bed nets, window screens, insecticides, etc
  • Infection control practices – Including proper hand hygiene, disinfecting surfaces in healthcare settings and food preparation areas, safely preparing and cooking foods, etc
  • Testing and treatment access – Particularly for STDs, HIV, and endemic diseases requiring systematic detection and drug therapy
  • Avoiding infection sites – Like staying away from others when acutely ill and avoiding travel to outbreak hotspots
  • Preparedness plans – Having response protocols, communication strategies, supply monitoring, and emergency care capacity in place for disease epidemics or pandemics

Additionally, addressing societal factors like poverty, discrimination, health literacy, housing, and access to care can help remove barriers that disproportionately expose marginalized groups to illness.

Most Dangerous Infectious Diseases

While illnesses like seasonal influenza impose a heavy annual burden, some less common infectious diseases stand out for their high case fatality rates when outbreaks occur. These deadlier infections include:

  • Rabies – Essentially 100% fatal once symptoms appear, but nearly 100% preventable by prompt vaccination after exposure
  • Ebola virus disease – Average 50% fatality rate, causes hemorrhagic bleeding and organ failure
  • Middle East Respiratory Syndrome (MERS) – 34% case fatality rate with pneumonia and organ failure
  • C. botulinum food poisoning – Toxin has up to 65% mortality, causes paralysis
  • Eastern equine encephalitis (EEE) – 30% mortality, severe brain inflammation
  • Anthrax – Gastrointestinal form has 25-75% mortality if untreated
  • CCHF viral hemorrhagic fever – Up to 40% fatality rate
  • C. tetani tetanus – 10-20% mortality if the severe infection causes painful muscle spasms

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