An infection occurs when harmful microorganisms enter the body, multiply, and cause damage to tissues or organs. This process often leads to symptoms that indicate the body’s response to these invading agents. Understanding how these microorganisms operate provides insight into the nature of infectious diseases.
The Agents of Infection
Infections are caused by various types of microorganisms. Bacteria are single-celled organisms found in different shapes, including spherical cocci, rod-shaped bacilli, and spiral forms. They reproduce by dividing into two identical cells.
Viruses are not considered living organisms because they require a host cell to replicate. A virus consists of genetic material, either DNA or RNA, encased within a protective protein shell called a capsid. Some viruses also possess an outer lipid membrane, or envelope.
Fungi include yeasts and molds. They can cause a range of infections, often affecting the skin and mucous membranes, such as athlete’s foot and yeast infections.
Parasites are organisms that live on or inside a host, deriving nutrients from it. This category includes single-celled protozoa, like those causing malaria, and larger organisms such as parasitic worms. Lice and mites are examples of external parasites that infest the skin.
How Infections Spread
Infectious agents travel from a source to a susceptible host through various pathways. Direct contact transmission involves physical contact between an infected individual and another person. This can occur through touching, kissing, sexual contact, or contact with bodily fluids, such as in the spread of the common cold or sexually transmitted infections.
Indirect contact transmission happens when an infectious agent is transferred via contaminated inanimate objects. Surfaces like doorknobs, shared toys, or eating utensils can harbor pathogens that are then picked up by another person. Norovirus, for example, can spread this way, lingering on surfaces.
Droplet transmission involves the spread of respiratory droplets expelled during activities like coughing, sneezing, or talking. These droplets usually travel short distances before falling out of the air. Diseases such as influenza, the common cold, and COVID-19 can spread via these droplets.
Airborne transmission involves smaller particles that remain suspended in the air for longer periods and can travel over greater distances. Pathogens causing measles or tuberculosis spread through airborne particles. Poor ventilation can enhance the spread of such agents indoors.
Vector-borne transmission occurs when living organisms, such as mosquitoes, ticks, or fleas, transmit infectious agents between hosts. These vectors acquire the pathogen from an infected animal or person and transfer it through bites. Malaria, carried by mosquitoes, and Lyme disease, transmitted by ticks, are examples.
Food and waterborne transmission occurs from ingesting food or water contaminated with pathogens. This often happens when food is improperly handled, cooked, or stored, or when water sources are contaminated with fecal matter. Salmonella from undercooked poultry or cholera from contaminated water are examples.
How Pathogens Enter the Body
Pathogens gain access to the body through specific entry points. The respiratory tract is a common route, where infectious particles or droplets are inhaled through the nose and mouth, reaching the throat and lungs. The body’s natural defenses, like mucus and cilia, work to trap and expel invaders, but some pathogens can overcome these barriers.
The gastrointestinal tract is an entry point when contaminated food or water is ingested. Pathogens enter through the mouth, travel down the esophagus, and reach the stomach and intestines. To establish an infection, they must survive the stomach’s acidic environment and digestive enzymes.
The skin and mucous membranes provide entry points for pathogens. While intact skin acts as a protective barrier, breaks such as cuts, scrapes, or insect bites can allow microorganisms to enter. Mucous membranes, found in areas like the eyes, nose, mouth, and genitals, are less protective and allow direct entry upon contact with infectious agents.
The urogenital tract is a pathway, particularly for sexually transmitted infections. Pathogens can enter through direct contact with mucous membranes during sexual activity. Infections can also occur if natural defenses in the urinary and reproductive systems are compromised.
The parenteral route involves the direct entry of pathogens into the bloodstream, bypassing the skin’s protective barrier. This can occur through needle sticks, contaminated blood transfusions, or insect bites that puncture the skin.
The Steps of Infection
Once a pathogen enters the body, a series of internal events unfold to establish an infection. The initial step is adhesion, where pathogens bind to specific host cells or tissues. They use specialized surface structures to attach to receptor molecules on the host cell’s surface.
Following adhesion, pathogens proceed to invasion, where they enter host cells or tissues. Some pathogens directly penetrate cells, while others might manipulate the host cell’s processes, inducing the cell to engulf them. This allows the pathogen to establish itself within the host.
After invading, pathogens begin multiplication, reproducing within the host. Bacteria multiply by dividing, increasing their numbers rapidly. Viruses, being obligate intracellular parasites, hijack the host cell’s machinery to produce new viral particles. This process consumes host resources and can lead to cellular dysfunction.
The final stage, pathogenesis, involves the mechanisms by which pathogens cause damage to host cells or tissues, leading to disease symptoms. Pathogens can produce toxins, harmful substances that interfere with host cell functions or directly destroy cells. They can also cause direct cell destruction through their replication, or trigger an overreaction from the host’s immune system that inadvertently harms the body. The body’s immune system attempts to counteract these effects, working to eliminate invaders and repair damage.