Short answers, pulled from the story.
An infection is the invasion of tissues by pathogens, their multiplication, and the host tissue's reaction to the infectious agent and its toxins. An infectious disease is the illness that results from an infection, but the two are not synonymous, since some infections cause no illness at all. Less than 5 percent of people infected with polio develop disease.
Infections are caused by bacteria, viruses and subviral agents such as viroids and prions, fungi, parasites, and arthropods. Examples include Mycobacterium tuberculosis, HIV, the fungus Candida, malaria, and parasitic worms like tapeworms and flukes. Arthropods such as ticks, mites, fleas, and lice cause disease usually termed infestation rather than infection.
Infectious diseases caused approximately 9.2 million deaths in 2013, which was 17 percent of all deaths that year. An estimated 1,680 million people died of infectious diseases in the 20th century. In 2021, COVID-19 directly caused 8.7 million deaths.
Diagnosis of infectious disease usually begins with medical history and physical examination, and many minor infections are treated without identifying the specific agent. Deeper methods include microbial culture, microscopy with stains such as the Gram stain and acid-fast stain, biochemical and serological tests, and polymerase chain reaction. Metagenomic sequencing is being developed to identify almost any pathogen in a single test.
Treatment depends on the type of pathogen, using antibiotics for bacteria, antivirals for viruses, antifungals for fungi, antiprotozoals for protozoa, and antihelminthics for parasitic worms. Antibiotics work only on bacteria and do not affect viruses. The most common antibiotic classes include penicillin, cephalosporins, aminoglycosides, macrolides, quinolones, and tetracyclines.
The Greek historian Thucydides, who lived around 460 to 400 BCE, was the first to write that diseases could spread from an infected person to others, in his account of the plague of Athens. The Persian physician Ibn Sina proposed a basic contagion theory in The Canon of Medicine in 1025. Louis Pasteur later proved that certain diseases are caused by infectious agents.