Infection is defined as the invasion of body tissues by pathogens or disease-causing agents and their proliferation resulting in reaction of host tissues.
The reaction could be to the organisms or toxins they produce.
The illness so resulted in known as infection or infectious disease. Most of the infectious diseases are can be transmitted from one person to other and are known as transmissible disease or communicable disease.
Infection can be transmitted in a variety of ways like skin contact, bodily fluids, feces, airborne particles, and touching a contaminated object.
Mode of transmission of disease depends on the agents.
Apart from above mentioned mode, infectious agents also spread through other organism called vector.
For example, mosquito is a vector How an infection spreads and its effect on the human body depend on the type of agent.
These infectious organisms are known as pathogens.
Bacteria, viruses, fungi, protozoa, parasites, and prions are different types of pathogen. Different pathogens vary in their size, shape, function, genetic code, and their mechanism of action on the body.
The severity of an infection can vary from being mild and barely noticeable [mild common cold] to be severe and life-threatening, and some even being resistant to treatment.
The body responds to defend itself from these pathogens by immune system. It may be able to contain kill the pathogen or the pathogens may grow too large for the immune system to fight and causing harm to body.
As infection is transmissible, it can spread to many persons from one persons. Some infections become a very big social threat due to rapid spread.
A virus is a small infectious agent that replicates only inside the living cells of other organisms. There are million of viruses and only few thousands have been identified so far.
Viruses typically are a l piece of genetic code protected by a coat of protein and fat.
Viruses attach to a cell and release genetic material which forces the cell to replicate, and the virus multiplies. Death of cell releases new virus which infect new cells.
Some viruses change the function of the host cell rather than killing them. Thus viruses such as human papillomavirus (HPV) can cause unrestrained cell division and thus cause cancer.
Different viruses target different cells. For example some are specific for repiratory infections others cause genital infections and so on.
Different viruses may cause infection in different age groups.
Some viruses like herpes are known to remain dormant for a long time and multiply when immunity weakens.
Here are some examples of viral infections:
- Common cold, mainly caused by the rhinovirus, coronavirus, and adenovirus
- Warts Human papillomaviruses (HPV) and herpes simplex virus (HSV)
gastroenteritis, caused by the rotavirus
- human immunodeficiency virus (HIV)
- hepatitis C
- Dengue fever
- H1N1 swine flu
Treatment of most of viral infections in symptomatic and supportive. Antiviral medications help in viruses like HIV, Herpes etc
Antibiotics are not effective against viruses.
Bacteria are single-celled microorganisms known as prokaryotes. A prokaryote is a unicellular organism that lacks a membrane-bound nucleus, mitochondria, or any other membrane-bound organelle.
According to estimates, there are one nonillion  bacteria on Earth.
Different bacteriae are of different shapes but spherical, rod shaped and spiral bacteria are most common shapes of known bacteriae.
These pathogens are known to thrive in almost any kind of environment, even extreme heat and cold.
Only few of the bacteriae cause diseases in humans.
Many of the bacteriae are also found in human body [for example gut] and live in harmony. Some of them may even defend from invading bacteriae.
Cholera, diphtheria, bubonic plague, tuberculosis, typhoid etc are some examples of bacterial diseases.
Bacterial infections can be treated with antibiotics but development of resistance is a common problem.
A fungus is eukaryote organism [cells contain membrane-bound organelles and clearly defined nuclei] belonging to kingdom fungi.
Yeast is a type of fungus. Fungi are commonly known because of skin and nail infections.
But they also cause systemic infections typically in persons with low immunity.
The structure of a fungus is normally long and cylindrical with small filaments branching from the main body. This structure is known as hypha.
They almost always reproduce through the spreading of single-celled spores,
Fungal skin infections are common especially in warm and moist areas like armpit or groin.
Fungal infections, especially systemic are seen in people with low immunity. Examples are HIV or AIDS, diabetes, chemotherapy etc
Athlete’s foot and ringworm are examples of fungal infection
A parasite is an organism that lives in another organism – the host – often harming it in the process. Some parasites don’t noticeably affect their hosts. Others grow, reproduce, or invade organ systems that make their hosts sick, resulting in a parasitic infection.
Examples of parasitic diseases are malaria, amoebiasis, giardiasis, hook works, tape worms etc.
Protozoa like amoeba and giardia are single-celled organisms that can live and multiply inside the body. Malarial parasite is also a protozoa.
Helminths are multi-celled organisms that can live in or outside of human body. They’re more commonly known as worms.
Spread of parasitic diseases can occur by contaminated water, food, waste, soil, and blood. Some can be passed through sexual contact. Some parasites are spread by insects that act as a vector, or carrier, of the disease
Persons who have a compromised immune system or are already sick with another illness are more prone. Sanitation plays a great role in checking parasitic infection. Measures like clean water, anti-mosquito sprays are quite effective.
The treatment of parasitic infections depend on the diagnosis.
For example, antimalarial drugs are prescribed for malaria.
A prion is a protein that contains no genetic material. It is normally harmless, but if it folds into an abnormal shape, it can affect the brain or other parts of the nervous system by triggering abnormal behavior in the body’s cells and proteins.
Prion diseases are rare, but often fatal.
Bovine spongiform encephalopathy or mad cow disease and Creutzfeldt-Jakob disease are examples of prion disease.