An infection occurs when microorganisms, such as bacteria, viruses, fungi, or parasites, invade and multiply within the body. These invaders can disrupt normal body functions and lead to illness.
How Pathogens Find a Way In
Pathogens must first contact a host and gain entry to initiate an infection. Direct contact transmission occurs through physical interaction, such as touching, kissing, or sexual contact. Indirect contact involves transmission via contaminated inanimate objects, called fomites, like doorknobs or shared utensils.
Pathogens can also travel through the air. Droplet transmission happens when an infected individual coughs or sneezes, expelling small droplets containing pathogens that can land on another person’s mucous membranes. Airborne transmission involves even smaller particles that remain suspended in the air for longer periods and can travel further, such as with measles or tuberculosis. Vehicle transmission refers to pathogens spread through contaminated food, water, or blood. Additionally, vectors like insects or other animals can transmit pathogens through bites or by carrying them on their bodies.
Once transmitted, pathogens must find a portal of entry to access the body’s internal environment. Common entry points include the respiratory tract, where pathogens are inhaled. The gastrointestinal tract serves as an entry point when contaminated food or water is ingested. Other pathways include the urogenital tract, breaks in the skin from cuts or abrasions, and mucous membranes found in the eyes.
Taking Hold: Pathogen Establishment and Growth
After gaining entry, pathogens must establish a foothold within the host to cause an infection. The initial step is adhesion, where pathogens attach to host cells or tissues. They achieve this using specific molecules called adhesins, which prevent them from being flushed away by natural body mechanisms like peristalsis or fluid flow.
Following successful adhesion, pathogens begin the process of colonization, multiplying at the site of entry to form a stable population. This involves overcoming localized host defenses that attempt to clear the invading microorganisms. For example, bacteria might form biofilms, which are communities encased in a protective matrix, enhancing their survival and resistance to host defenses and even antibiotics.
Pathogens then multiply and increase their numbers within the host. This reproduction can occur either intracellularly, where pathogens invade and replicate inside host cells, or extracellularly, where they multiply in body fluids and tissues. For an infection to progress, a sufficient number of pathogens, known as the infectious dose, is often required to overwhelm the host’s immune system.
The Body’s Battle: Immune Response and Evasion
The body immediately mounts a defense when pathogens establish themselves. The host’s defenses involve both innate and adaptive immune responses. Innate immunity provides an immediate, non-specific response, utilizing mechanisms such as inflammation and phagocytes, which are cells that engulf and destroy foreign particles. This initial response aims to contain the infection rapidly.
As the infection progresses, the adaptive immune system becomes active, offering a more specific and memory-based response. This involves the production of antibodies that target specific pathogens and the activation of T-cells that can directly kill infected cells or coordinate other immune responses. The adaptive response provides long-lasting protection against previously encountered pathogens.
Pathogens have evolved strategies to evade or suppress host immune responses. Some bacteria produce capsules, outer layers that prevent them from being recognized and engulfed by phagocytes. Others employ antigenic variation, constantly changing their surface proteins to avoid detection by existing antibodies or immune cells.
Certain pathogens can survive by hiding inside host cells, a strategy known as intracellular survival, which conceals them from immune surveillance. Pathogens may also produce enzymes that break down components of the immune system or host tissues, interfering with the body’s defense mechanisms. Some pathogens also directly interfere with immune cell function, suppressing the host’s ability to fight off the infection. The outcome of this interaction, whether the infection is cleared or progresses to illness, depends on the balance between the pathogen’s ability to evade and the host’s immune strength.
When Symptoms Appear: The Signs of Infection
The culmination of pathogen establishment and the immune response often manifests as observable signs and symptoms of illness. Symptoms arise primarily from the pathogen’s direct damage to host cells and tissues. This damage can occur through various mechanisms, including the release of toxins that poison cells, enzymes that break down tissue, or direct lysis, where pathogens cause cells to burst.
The host’s own immune response, while protective, can also contribute to the symptoms experienced during an infection. Inflammation, a part of the immune response, can lead to redness, swelling, heat, and pain at the site of infection. Fever, fatigue, and general aches are common systemic symptoms that result from the body’s widespread immune activation as it attempts to eliminate the threat.
Symptoms can be general or highly specific, depending on the affected body system. For example, a respiratory infection might cause a cough and sore throat, while a gastrointestinal infection could lead to diarrhea and abdominal cramps. The time between initial exposure to a pathogen and the appearance of symptoms is known as the incubation period. This period varies significantly depending on the pathogen and host factors, ranging from hours for some foodborne illnesses to weeks or even years for others.