Infectious diseases are caused by microorganisms.

A microorganism that causes an infectious disease is known as a pathogen, or more commonly, a germ.

Each infectious disease involves a relationship between a pathogen and a host, the organism that the pathogen infects.

Some infectious diseases are contagious diseases; the pathogen must be transmitted directly from one human being to another.

Others are vector diseases - the pathogen needs a carrier, or vector -to transfer it from one human to another.

Bacteria that Cause Syphilis Treponema pallidumSyphilis

Syphilis is a contagious disease. It caused by a bacterium known as Treponema pallidum, which dies within seconds if it is exposed to air and light. Therefore, it must be transmitted directly from one person to another.

Treponema pallidum can live and multiply in the human body, until it inhabits the entire body.

There are three stages of syphilis.

In the first stage, there is a small open sore at the point of original infection. In this stage, the bacteria can easily be transmitted if the sore touches any moist membrane of another person.

The second stage usually begins two to six months after the first sore has appeared. Symptoms of the second stage are rashes, blotches, and sores on the skin and mucus membranes.

After this stage, there may be no symptoms for 10 to 20 years. At this time, the treponemes are attacking internal body parts, especially the nerves and brain.

During the third stage, blindness, deafness or insanity may occur.

Anopheles Mosquito Feeding on HumanMalaria

Pathogen that Causes Malaria PlasmodiumMalaria is a vector disease that is carried by Anopheles mosquitoes.

The pathogen that causes malaria is a eukaryote of the genus Plasmodium.

When a mosquito that carries this pathogen bites a human being, the pathogen enters the bloodstream of its human host.

It then travels to the bloodstream to the liver, where it multiplies.

The offspring then return to the bloodstream.

They enter the red blood cells and multiply, in a process that destroys the red blood cells.

As the blood cells are destroyed, the symptoms of malaria, which include fever, chills, vomiting and convulsions, develop.

Sometimes, male and female forms of Plasmodium are produced.

If the human host is bitten by a mosquito of the genus Anopheles, the male and female forms may be picked up by the insect.

In the mosquito's stomach, the male form will unite with the female, forming a single new individual, which squeezes through the mosquito's stomach wall and begins dividing.

Eventually, a cluster of hundreds of individual Plasmodia will be formed within the mosquito.

These individuals break free from the cluster and travel to the mosquito's salivary glands.

When the mosquito bites another person, it injects its saliva, along with the Plasmodia, into that person's blood.

Host Specificity

Some pathogens are highly host-specific. This means that they infect only a few kinds of organisms.

For example, the pathogen that causes malaria infects only human beings and Anopheles mosquitoes.

Many pathogens are not highly host-specific. They will infect a number of different types of living things.


Virulence is the degree to which a pathogen can affect its host.

Different factors can determine how virulent a pathogen is.

For example, in diphtheria, the host develops symptoms because of a toxin produced by the bacterium Corynebacterium diphtheriae.  Different strains of this bacteria produce different amounts of this toxin, even when they are grown in a petri dish under identical conditions.

The virulence of a virus can depend on its ability to replicate while the host's immune system is fighting it.

Resistance and Immunity

Resistance is the ability of a host to fight a pathogen.

A pathogen might kill a host with low resistance, cause severe illness in someone with medium resistance and not cause symptoms at all in a host with high resistance.

The ability to completely avoid the symptoms of a disease is known as immunity.

Different individuals of the same host species can have different degrees of resistance.

Resistance can be inherited. We can develop breeds of domestic plants and animals that are more resistant to certain diseases than other breeds.

Resistance can also be acquired.

When a pathogen infects a host, the host reacts by producing antibodies. These are proteins that identify and neutralize the pathogen.

If the host survives the infection, its body retains the ability to produce antibodies against it.

Once these antibodies are produced, if the same kind of pathogen infects it again, the host can act immediately against it.

Immunity can be achieved naturally, and it can be created artificially through vaccination.

Edward JennerSmallpox Vaccine

In the late eighteenth century, the British physician Edward Jenner noticed that milkmaids and other people who worked with cows rarely developed smallpox, a fatal disease that was common at the time.

These workers had usually been infected by cowpox, a mild disease of cattle.

Jenner believed that if he deliberately infected a human being with cowpox, he might be able to make them immune to smallpox.

This led him to develop the practice of vaccination.

On May 14, 1796, Jenner gave the first smallpox vaccine to an eight-year-old boy named James Phipps.

Smallpox was completely eradicated by the end of 1979.