How Bacteria and Viruses cause Infection

Both bacteria and viruses can cause disease. However, these two types of infectious agents achieve their pathogenicity in different ways. Infection itself simply means that a pathogen has entered the body, but does not necessarily equate with disease, since a person can be infected by an agent, but never show signs of illness. Here is a summary of how bacteria and viruses enter the body and cause infection.

* How Bacteria Infect Human Hosts *

Some bacterial are full-time pathogens, others, such as normal flora, only cause disease when they are introduced into a normally sterile area of the human body or when they are able to overpopulate because the human immune system is not working as well as it should.

* How Bacteria Enter the Body *

When bacteria infect humans, they do so by entering the body and finding a suitable place to grow. Depending on the type of bacteria, the pathogen may enter through contact with the mucus membrane or breaks in the skin, such as cuts or other trauma.

* How Bacterial Infections Begin *

Once a pathogenic bacterium is inside, unchecked it can grow and spread. Most bacterially-mediated infectious diseases begin as small localized infections, and, if they spread, do so by gaining access to the blood stream.

* How Bacteria Make a Person Feel Sick *

The pathogenicity of invasive bacteria (the factors that make a person sick) is often the result of toxins that the bacteria produce. These toxins are simply byproducts of the bacterium’s metabolism that, when present in the human body, interfere with normal functioning.

* Bacterial Antigens and the Immune System *

Since some of the structural components and metabolic products of bacteria are not normally found in the human body, the body’s immune system can recognize these antigenic (foreign) particles on the bacteria and launch an immune response to try and eliminate the invader. Surprisingly, many of the symptoms that make a person suffer during an infection (bacterial or viral) actually result from the activities of the immune system trying to rid the body of infection.

But elimination of a pathogen is easier said than done. Many bacteria have ‘weapons’, such as capsules, fimbriae and other virulence factors that help them cause infection and hide them from cells of the immune system.

* How Viruses Infect Human Hosts *

Viruses are also picked up from the environment. When a virus happens to collide with a cell that it can infect, it inserts its genome into the host cell and the infected cell is then directed to make more viruses.

* Viral Antigenic Particles and Markers *

Viruses also have antigenic particles which can be recognized as foreign by cells of the human immune system. When viruses are found by patrolling cells of the immune system, the infectious particle can be often destroyed. Although there are some bacteria that actually enter cells of the body during infection, all viruses must enter a cell in order to reproduce, and once inside the host cell, a virus is difficult to eliminate.

Animal cells that have been infected by a virus display viral markers on their surface; a calling card that the virus left when entering the cell. These marked cells can be recognized as infected by T-cells of the immune system and are sometimes destroyed.

* Latent Viruses *

Some viruses become latent after infecting a cell. Latency is achieved when a DNA virus insert its genome into the genome of the host. In this state it is called a provirus and is permanently part of the host cell’s DNA. The varicella zoster virus, which causes chickenpox and can reemerge as shingles, is an example of a provirus.

* Sources*

Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings

Microbe World: Meet the Microbes – Virus or Bacterium

The Virtual Microbiology Classroom

CDC Meningitis Homepage