Infectious Diseases

A healthy person lives in harmony with the microbial flora that helps protect its host from invasion by pathogens, usually defined as microorganisms that have the capacity to cause disease. The microbial flora is mostly bacteria and fungi and includes normal resident flora, which is present consistently and which promptly reestablishes itself if disturbed, and transient flora, which may colonize the host for hours to weeks but does not permanently establish itself. Organisms that are normal flora can occasionally cause disease, especially when defenses are disrupted.

Laboratory tests may identify organisms directly (eg, visually, using a microscope, growing the organism in culture) or indirectly (eg, identifying antibodies to the organism). General types of tests include

Immunity can be achieved

Bacteria can be classified by their need and tolerance for oxygen:

Human parasites are organisms that live on or in a person and derive nutrients from that person (its host). There are 3 types of parasites:

Arbovirus (arthropod-borne virus) applies to any virus that is transmitted to humans and/or other vertebrates by certain species of blood-feeding arthropods, chiefly insects (flies and mosquitoes) and arachnids (ticks). Arbovirus is not part of the current viral classification system, which is based on the nature and structure of the viral genome.

Bacteria are microorganisms that have circular double-stranded DNA and (except for mycoplasmas) cell walls. Most bacteria live extracellularly. Some bacteria (eg, Salmonella typhi; Neisseria gonorrhoeae; Legionella, Mycobacteria, Rickettsia, Chlamydia, and Chlamydophila species) preferentially reside and replicate intracellularly. Some bacteria such as chlamydiae, Chlamydophila species, and rickettsiae are obligate intracellular pathogens (ie, able to grow, reproduce, and cause disease only within the cells of the host). Others (eg, Salmonella typhi, Brucella species, Francisella tularensis, N. gonorrhoeae, Neisseria meningitidis, Legionella and Listeria species, Mycobacterium tuberculosis) are facultative intracellular pathogens.

Tapeworms (cestodes) are flat, parasitic worms. The four main intestinal cestode pathogens of humans are

Three species of Chlamydia cause human disease, including sexually transmitted infections and respiratory infections. All are susceptible to macrolides (eg, azithromycin), tetracyclines (eg, doxycycline), and fluoroquinolones.

COVID-19 is an acute, sometimes severe, respiratory illness caused by a novel coronavirus SARS-CoV-2.

Enteroviruses, along with rhinoviruses (see Common Cold) and human parechoviruses, are a genus of picornaviruses (pico, or small, RNA viruses). All enteroviruses are antigenically heterogeneous and have wide geographic distribution.

Free-living amebas are protozoa that live independently in soil or water and do not require a human or animal host. They rarely cause disease, in contrast to the parasitic ameba Entamoeba histolytica, which is a common cause of intestinal infection (amebiasis). Pathogenic free-living amebas are of the genera Acanthamoeba, Balamuthia, Naegleria, and Sappinia.

Fungal infections are often classified as either

Gram-negative bacilli are responsible for numerous diseases. Some are commensal organisms present among normal intestinal flora. These commensal organisms plus others from animal or environmental reservoirs may cause disease.

Acinetobacter species are gram-negative organisms that can cause suppurative infections in any organ system; these bacteria are often opportunists in hospitalized patients.

Enterococci are gram-positive, facultative anaerobic organisms. Enterococcus faecalis and E. faecium cause a variety of infections, including endocarditis, urinary tract infections, prostatitis, intra-abdominal infection, cellulitis, and wound infection as well as concurrent bacteremia.

Anthrax is caused by the gram-positive Bacillus anthracis, which are toxin-producing, encapsulated, facultative anaerobic organisms. Anthrax, an often fatal disease of animals, is transmitted to humans by contact with infected animals or their products. In humans, infection is typically acquired through the skin. Inhalation infection is less common; oropharyngeal, meningeal, and gastrointestinal infections are rare. For inhalation and gastrointestinal infections, nonspecific local symptoms are typically followed in several days by severe systemic illness, shock, and often death. Empiric treatment is with ciprofloxacin, levofloxacin, moxifloxacin, or doxycycline. A vaccine is available.

Eight types of herpesviruses infect humans (All.see table Herpesviruses That Infect Humans). After initial infection, all herpesviruses remain latent within specific host cells and may subsequently reactivate. Clinical syndromes due to primary infection can vary significantly from those caused by reactivation of these viruses. Herpesviruses do not survive long outside a host; thus, transmission usually requires intimate contact. In people with latent infection, the virus can reactivate without causing symptoms; in such cases, asymptomatic shedding occurs and people can transmit infection.

Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain infections and cancers. Initial infection may cause nonspecific febrile illness. Risk of subsequent manifestations—related to immunodeficiency—is proportional to the level of CD4+ lymphocyte depletion. HIV can directly damage the brain, gonads, kidneys, and heart, causing cognitive impairment, hypogonadism, renal insufficiency, and cardiomyopathy. Manifestations range from asymptomatic carriage to acquired immune deficiency syndrome (AIDS), which is defined by serious opportunistic infections or cancers or a CD4 count of 200/mcL. HIV infection can be diagnosed by antibody, nucleic acid (HIV RNA), or antigen (p24) testing. Screening should be routinely offered to all adults and adolescents. Treatment aims to suppress HIV replication by using combinations of ≥ 2 drugs that inhibit HIV enzymes; treatment can restore immune function in most patients if suppression of replication is sustained.

The most important intestinal protozoan pathogens are

Tuberculosis (TB) is a chronic, progressive mycobacterial infection, often with a period of latency following initial infection. TB most commonly affects the lungs. Symptoms include productive cough, fever, weight loss, and malaise. Diagnosis is most often by sputum smear and culture and, increasingly, by rapid molecular-based diagnostic tests. Treatment is with multiple antimicrobial drugs given for at least 6 mo.

Angiostrongyliasis is infection with larvae of worms of the genus Angiostrongylus. Depending on the infecting species, abdominal symptoms (Angiostrongylus costaricensis) or neural involvement with eosinophilic meningitis (A. cantonensis) result. Diagnosis is suspected by history and presence of eosinophilia in cerebrospinal fluid (CSF) and/or blood. Anthelmintics are either ineffective or deleterious.

Monkeypox, a rare disease, is caused by the monkeypox virus, which is structurally related to the smallpox virus and causes similar, but usually milder illness.

Viral infections commonly affect the upper or lower respiratory tract. Although respiratory infections can be classified by the causative virus (eg, influenza), they are generally classified clinically according to syndrome (eg, the common cold, bronchiolitis, croup, pneumonia). Although specific pathogens commonly cause characteristic clinical manifestations (eg, rhinovirus typically causes the common cold, respiratory syncytial virus [RSV] typically causes bronchiolitis), each can cause many of the viral respiratory syndromes (All.see table Causes of Common Viral Respiratory Syndromes).

Rickettsial diseases (rickettsioses) and related diseases (anaplasmosis, ehrlichiosis, Q fever, scrub typhus) are caused by a group of gram-negative, obligately intracellular coccobacilli. All, except for Coxiella burnetii, have an arthropod vector. Symptoms usually include sudden-onset fever with severe headache, malaise, prostration, and, in most cases, a characteristic rash. Diagnosis is clinical, confirmed by immunofluorescence assay or polymerase chain reaction (PCR). Treatment is with tetracyclines or, except for anaplasmosis and ehrlichiosis, chloramphenicol.

Sexually transmitted diseases (STDs), also termed sexually transmitted infections (STIs), can be caused by a number of microorganisms that vary widely in size, life cycle, the diseases and symptoms caused, and susceptibility to available treatments.

Bejel, pinta, and yaws (endemic treponematoses) are chronic, tropical, nonvenereal spirochetal infections spread by body contact. Symptoms of bejel are mucous membrane and cutaneous lesions, followed by bone and skin gummas. Yaws causes periostitis and dermal lesions. Pinta lesions are confined to the dermis. Diagnosis is clinical and epidemiologic. Treatment is with penicillin.

Flukes are parasitic flatworms that infect the blood vessels, gastrointestinal tract, lungs, or liver. They are often categorized according to the principle organ system they invade:

Viruses are the smallest parasites, typically ranging from 0.02 to 0.3 micrometer, although several very large viruses up to 1 micrometer long (megavirus, pandoravirus) have recently been discovered. Viruses depend completely on cells (bacterial, plant, or animal) to reproduce. Viruses have an outer cover of protein and sometimes lipid, an RNA or DNA core, and sometimes enzymes needed for the first steps of viral replication.