Bacteria are single-celled microorganisms, also known as prokaryotes.

Experts estimate that there are at least 1 nonillion bacteria on Earth. A nonillion is a 1 followed by 30 zeros. Much of the Earth’s biomass comprises bacteria.

Bacteria take three main shapes:

• Spherical: These are known as cocci.
• Rod-shaped: These have the name bacilli.
• Spiral: Coiled bacteria are known as spirilla. If the coil of a spirillum is particularly tight, scientists call it a spirochete.

Bacteria can live in almost any type of environment, from extreme heat to intense cold, and some can even survive in radioactive waste.

There are trillions of strains of bacteria, and few cause diseases in humans. Some of them live inside the human body, such as in the gut or airways, without causing harm.

Some “good” bacteria attack “bad” bacteria and prevent them from causing sickness. However, some bacterial diseases can be deadly.

Viral infections occur due to infection with a virus. Viruses contain a small piece of genetic code, and a coat of protein and lipid (fat) molecules protects them. Viruses invade a host and attach themselves to a cell. As they enter the cell, they release their genetic material. This material forces the cell to replicate the virus, and the virus multiplies. When the cell dies, it releases new viruses, which infect new cells.

Not all viruses destroy their host cell, however. Some of them change the function of the cell. Some viruses, such as human papillomavirus (HPV) and Epstein-Barr virus (EBV), can lead to cancer by forcing cells to replicate in an uncontrolled way. A virus can also target certain age groups, such as infants or young children. Viruses may remain dormant for a period before multiplying again. The person with the virus can appear to have fully recovered, but they may get sick again when the virus reactivates.

A fungus is often a multicellular parasite that can decompose and absorb organic matter using an enzyme. However, some types, such as yeasts, are single celled. Fungi almost always reproduce by spreading single celled spores. The structure of a fungus is usually long and cylindrical, with small filaments branching from the main body. There are approximately 5.1 million species of fungus.

Many fungal infections develop in the upper layers of the skin, and some progress to the deeper layers. Inhaled yeast or mold spores can sometimes lead to fungal infections, such as pneumonia, or infections throughout the body. These are also known as systemic infections. The body usually has a population of good bacteria that help maintain the balance of microorganisms.

These line the intestines, mouth, vagina, and other parts of the body. Those with a higher risk of developing a fungal infection include people who: use antibiotics for a long time have a weakened immune system, due, for example, to living with HIV or diabetes or receiving chemotherapy treatment have undergone a transplant, as they take medications to prevent their body from rejecting the new organ.

Prions are misfolded proteins with the ability to transmit their misfolded shape onto normal variants of the same protein.

They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It is not known what causes the normal protein to misfold, but the abnormal three-dimensional structure is suspected of conferring infectious properties, collapsing nearby protein molecules into the same shape. The word prion derives from "proteinaceous infectious particle".

The hypothesized role of a protein as an infectious agent stands in contrast to all other known infectious agents such as viruses, bacteria, fungi, and parasites, all of which contain nucleic acids (DNA, RNA, or both). 

A parasite is an organism that lives within or on a host. The host is another organism. The parasite uses the host’s resources to fuel its life cycle. It uses the host’s resources to maintain itself. Parasites vary widely. Around 70 percent are not visible to the human eye, such as the malarial parasite, but some worm parasites can reach over 30 meters in length.

Parasites are not a disease, but they can spread diseases. Different parasites have different effects.

Endoparasite

These live inside the host. They include heartworm, tapeworm, and flatworms. An intercellular parasite lives in the spaces within the host’s body, within the host’s cells. They include bacteria and viruses.

Endoparasites rely on a third organism, known as the vector, or carrier. The vector transmits the endoparasite to the host. The mosquito is a vector for many parasites, including the protozoan known as Plasmodium, which causes malaria.

Epiparasite

These feed on other parasites in a relationship known as hyper parasitism. A flea lives on a dog, but the flea may have a protozoan in its digestive tract. The protozoan is the hyperparasite.

Your skin is the largest organ of your body. Its function is to protect your body from infection. Sometimes the skin itself becomes infected. Skin infections are caused by a wide variety of germs, and symptoms can vary from mild to serious. Mild infections may be treatable with over-the-counter medications and home remedies, whereas other infections may require medical attention.

Eye infections occur when some harmful organism invades an area of your eye. The source of the infection is often bacterial or viral, but it may be due to some other cause. Treatment depends on the cause of the infection. Despite the cause, the symptoms that occur when you have an eye infection are typically similar. If you suspect you have an eye infection, have your physician evaluate the problem.

There can be numerous types of eye infections that can include conjunctivitis, keratitis, acanthamoeba keratitis, blepharitis, uveitis, cellulitis, ocular herpes, endophthalmitis, chlamydia trachomatis, a sty, dacryostenosis corneal ulcers, and orbital cellulitis. When it comes to dealing with potential eye infections, prevention is the best cure.

A urinary tract infection, or UTI, is an infection in any part of your urinary system, which includes your kidneys, bladder, ureters, and urethra. If you are a woman, your chance of getting a urinary tract infection is high. Some experts rank your lifetime risk of getting one as high as 1 in 2, with many women having repeat infections, sometimes for years. About 1 in 10 men will get a UTI in their lifetime.

Viruses and microorganisms sometimes invade the body, infecting various organs and causing everything from mild disturbances to serious problems. Bacterial organisms are often to blame, but animal parasites and fungi can also cause infection. Neurological infections occur when these viruses and organisms invade the nervous system.

Symptoms of Infection

Pain, swelling, redness, impaired function and fever are all characteristics of an infection. There may also be heat at the site of the infected area. In the case of some viral infections, drowsiness, confusion, and convulsions may occur.

Tropical diseases are diseases that are prevalent in or unique to tropical and subtropical regions. The diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, which controls the insect population by forcing hibernation. However, many were present in northern Europe and northern America in the 17^th and 18^th centuries before modern understanding of disease causation.

The initial impetus for tropical medicine was to protect the health of colonial settlers, notably in India under the British Raj. Insects such as mosquitoes and flies are by far the most common disease carrier, or vector. These insects may carry a parasite, bacterium or virus that is infectious to humans and animals. Most often disease is transmitted by an insect "bite", which causes transmission of the infectious agent through subcutaneous blood exchange. Vaccines are not available for most of the diseases listed here, and many do not have cures.

Human exploration of tropical rainforests, deforestation, rising immigration and increased international air travel and other tourism to tropical regions has led to an increased incidence of such diseases to non-tropical countries.

Infection prevention and control (IPC) is a scientific approach and practical solution designed to prevent harm caused by infection to patients and health workers. It is grounded in infectious diseases, epidemiology, social science, and health system strengthening. IPC occupies a unique position in the field of patient safety and quality universal health coverage since it is relevant to health workers and patients at every single health-care encounter. No country, no health-care facility, even within the most advanced and sophisticated health-care systems, can claim to be free of the problem of health care-associated infections. The need for having IPC programmes nationally and at the facility level is clearly reinforced within the WHO 100 Core Health Indicators list.

Antibiotics are grouped into "families" of similar types. Bacteria also are put together in groups of similar types, such as streptococcus or E. coli. Certain types of bacteria are especially susceptible to particular classes of antibiotics. Treatment can be targeted more precisely if your doctor knows what type of bacteria you are infected with. Antibiotics are usually reserved for bacterial infections because these types of drugs have no effect on illnesses caused by viruses. But sometimes it is difficult to tell which type of germ is at work. For example, pneumonia can be caused by a bacterium, a virus, a fungus, or a parasite. The overuse of antibiotics has resulted in several types of bacteria developing resistance to one or more varieties of antibiotics. This makes these bacteria much more difficult to treat.

Postoperative infections are commonly seen in the obstetrics and gynaecology. These infections sometimes may be polymicrobial which means caused by several species of microorganisms. Early recognition and treatment can prevent a threat to the foetus.

Paediatric infectious diseases are those occur in the children from birth to teen years. Two-third of the deaths of children in past 10 years was caused by infectious diseases. New-borns are at higher risk as their immune system is immature and cannot fight infections. 

Pharmacology and Therapeutics is a source for advances in human therapeutics, transcends the drug discovery and development. It includes the study of pathophysiology of infectious diseases and mechanism of drug action, adverse drug effects, therapeutics and toxicology.

Vaccine is a biological preparation that supports immune system to fight particular diseases. It played a crucial role in saving lives and greatly changing the burden of many infectious diseases. Awareness and effectiveness of Vaccination has made many diseases rare over years.

Molecular Diagnostic Techniques developed a revolution in the diagnosing and monitoring pattern of infectious diseases. Advances in molecular diagnostic techniques over the past 10 years initiated new approach for microbial identification and characterization.

Public health is the discipline which deals with the science and art of preventing infections by improving the health of population.