Viruses
Replicate inside host cells. A protein coat surrounds their genetic material; some have an envelope. Examples: influenza and SARS-CoV-2.
Infections
A Constant Challenge
From common colds to serious disease, infections challenge the body’s defenses. Learn how the immune system recognizes and controls pathogens, how immune memory develops, and how prevention can reduce risk.
Explore the science →What Are Infections?
Infection occurs when a pathogen enters the body, establishes itself and multiplies. Infection can be asymptomatic or cause illness.
The outcome depends on the pathogen, the site of infection and the host response. Not all microbes are harmful, and not every exposure leads to infection.
Learn how defenses work →Common Pathogens
Types of Pathogens →Bacteria
Single-celled organisms; many are harmless or beneficial. Examples of pathogens: Streptococcus and M. tuberculosis.
Fungi
Yeasts and molds can cause superficial or invasive infections. Examples: Candida and Aspergillus.
Parasites
Protozoa and helminths may have complex life cycles. Examples: Plasmodium and intestinal worms.
Key Takeaways
- Different pathogens require different defenses.
- Many infections are controlled by a healthy immune response.
- Symptoms can reflect both pathogen damage and the host response.
- Vaccination and prevention reduce risk.
- Age and underlying conditions influence susceptibility.
How Your Immune System Fights Infections
A coordinated, multilayered defense.
1. Recognition
Innate receptors detect microbial patterns and tissue damage.
2. Early response
Local mediators recruit cells and coordinate an initial defense.
3. Activation
Antigen presentation and other signals activate specific T- and B-cell responses.
4. Elimination
Phagocytes, antibodies and effector T cells help remove pathogens and infected cells.
5. Memory
Some activated lymphocytes persist and respond more efficiently upon re-exposure.
Innate vs. Adaptive Immunity
Two complementary arms, working together.
Innate immunity
Fast and broad
- Acts within minutes to hours.
- Recognizes shared microbial patterns.
- Includes barriers, phagocytes and NK cells.
- Helps initiate adaptive responses.
Adaptive immunity
Specific and long-lasting
- Initial responses develop over days.
- Recognizes specific antigens.
- Antibodies and T cells provide specific defense.
- Can generate immune memory.
Common Infections Across the Lifespan
Patterns of exposure and disease severity change with age.
Infancy
Immune responses are developing. Maternal antibodies provide temporary protection, and vaccination begins early.
Childhood
Respiratory and gastrointestinal infections are common. Immune experience and vaccination build specific protection.
Adolescence
Exposure patterns change with school, social contact and activities. Recommended vaccines remain important.
Adulthood
Risk varies with exposure, occupation, travel, health conditions and vaccination status.
Older adulthood
Immune changes and chronic conditions can increase the severity of some infections.
Infections and Aging
Why some infections become more consequential.
- Changes in immune-cell function.
- Reduced diversity of some naive lymphocyte populations.
- Altered antibody and memory responses.
- Chronic inflammation and health conditions.
- Nutrition, exposure and physiological reserve.
Vaccination, timely assessment and appropriate treatment help reduce avoidable harm.
Learn about immune aging →Prevention and Protection
Practical steps that reduce infection risk.
Vaccination
Stay up to date with recommended vaccines.
Hygiene
Handwashing and food safety limit transmission.
Healthy habits
Sleep, nutrition and regular activity support overall health.
Reduce exposure
Use appropriate ventilation and protective measures.
Knowledge today.
Stronger tomorrows.
Understanding infections helps you make informed choices for yourself and the people around you.
Explore vaccination →Selected sources and references
Artwork illustrates the main mechanisms schematically; molecular sizes, colors and distances are not literal.