Overview of Infectious Agents: Chlamydia, Mycoplasmas, Parasites, Fungi, Viruses, and Prions

Chlamydia and Mycoplasmas

Infections Caused by Chlamydia and Mycoplasmas

Chlamydia and mycoplasmas are unique groups of bacteria that cause a variety of human diseases. Both are obligate intracellular pathogens, meaning they require host cells to replicate.

• Chlamydia: Small, Gram-negative bacteria that lack peptidoglycan in their cell walls. They are responsible for diseases such as Chlamydia trachomatis infections (e.g., trachoma, sexually transmitted infections).

• Mycoplasmas: The smallest free-living bacteria, lacking a cell wall entirely. Mycoplasma pneumoniae is a common cause of atypical pneumonia.

• Transmission: Often spread through direct contact, respiratory droplets, or sexual contact.

• Diagnosis: Involves nucleic acid amplification tests (NAATs), serology, and culture (for mycoplasmas).

Example: Chlamydia trachomatis is the most common bacterial cause of sexually transmitted infections worldwide.

Parasitic Infections

Protozoal (Unicellular) Infections

Protozoa are single-celled eukaryotic organisms that can cause a range of diseases in humans, often transmitted via contaminated water, food, or vectors.

• Common Protozoal Diseases: Malaria (Plasmodium spp.), amoebiasis (Entamoeba histolytica), giardiasis (Giardia lamblia), and trichomoniasis (Trichomonas vaginalis).

• Transmission: Fecal-oral route, insect vectors, or direct contact.

• Diagnosis: Microscopy, antigen detection, and molecular methods.

Example: Malaria is transmitted by the bite of infected Anopheles mosquitoes.

Metazoal (Multicellular) Parasitic Infections

Metazoal parasites include multicellular organisms such as roundworms (nematodes) and flatworms (platyhelminths). These parasites can infect various organs and cause significant morbidity.

• Roundworms: Ascaris lumbricoides, Enterobius vermicularis (pinworm).

• Flatworms: Includes flukes (Schistosoma spp.) and tapeworms (Taenia spp.).

• Transmission: Ingestion of eggs or larvae, skin penetration, or vector-borne.

Example: Schistosomiasis is caused by blood flukes and is prevalent in tropical regions.

Mycotic (Fungal) Infections

General Properties of Infectious Fungi

Fungi are eukaryotic organisms that can exist as yeasts, molds, or dimorphic forms. They cause a variety of infections, particularly in immunocompromised hosts.

• Superficial Mycoses: Affect the outermost layers of skin and hair (e.g., tinea versicolor).

• Cutaneous Mycoses: Involve the skin, hair, and nails (e.g., dermatophyte infections like athlete's foot).

• Subcutaneous Mycoses: Penetrate deeper into the skin, often following trauma (e.g., sporotrichosis).

• Systemic Mycoses: Affect internal organs, often life-threatening (e.g., histoplasmosis, cryptococcosis).

Diagnosis: Microscopy, culture, serology, and molecular methods.

Example: Candida albicans can cause superficial infections like oral thrush and systemic infections in immunocompromised patients.

Viruses

General Properties of Viruses

Viruses are acellular infectious agents composed of genetic material (DNA or RNA) surrounded by a protein coat. They require host cells for replication.

• Structure: Nucleic acid core, capsid, sometimes an envelope derived from host membranes.

• Replication: Attachment, penetration, synthesis, assembly, and release.

• Classification: Based on nucleic acid type, symmetry, presence of envelope, and host range.

Example: Influenza virus is an enveloped, segmented, negative-sense RNA virus.

Viruses Causing Human Diseases

Viruses are responsible for a wide range of human diseases, affecting various organ systems.

• Respiratory Viruses: Influenza, respiratory syncytial virus (RSV), rhinovirus.

• Enteric Viruses: Norovirus, rotavirus, enteroviruses.

• Viruses Causing Diarrhea: Rotavirus, norovirus, adenovirus (types 40/41).

• Exanthemas (Rashes): Measles, rubella, varicella-zoster virus.

• Glandular Enlargement: Mumps virus, Epstein-Barr virus (EBV).

• Hepatitis Viruses: Hepatitis A, B, C, D, and E viruses.

• CNS Infections: Herpes simplex virus, rabies virus, poliovirus.

• HIV/AIDS: Human immunodeficiency virus causes acquired immunodeficiency syndrome, leading to severe immune suppression.

Diagnosis: PCR, serology, antigen detection, and viral culture.

Prions

Infectious Proteins Affecting Human and Animal Populations

Prions are misfolded proteins that can induce abnormal folding of normal cellular proteins, particularly in the brain. They are unique infectious agents lacking nucleic acids.

• Diseases: Creutzfeldt-Jakob disease (CJD), variant CJD, kuru in humans; scrapie and bovine spongiform encephalopathy (BSE) in animals.

• Transmission: Ingestion of contaminated tissue, medical procedures, or inherited mutations.

• Prevention: Strict control of medical instruments, avoiding consumption of contaminated animal products, and surveillance of animal populations.

Example: BSE, or "mad cow disease," is a prion disease transmitted to humans as variant CJD through consumption of infected beef.