Nature of Infectious Diseases

Understanding Infectious Diseases Disease: the term refers to conditions that impair normal tissue function. Cystic fibrosis Atherosclerosis Measles All are diseases but there are fundamentally different causes for each disease

Infectious disease: a disease caused by the invasion of a host by agents whose activities harm the host’s tissues and can be transmitted to others (they are infectious) Pathogens: microorganisms that are capable of causing disease. A true pathogen is one that causes disease in virtually every susceptible host.

The terms “infections” and “disease” are not synonymous. An infection results when a pathogen invades and begin growing in the host A disease results only if tissue function is impaired by the invasion and growth of a pathogen. polio virus Ebola hemorrhagic fever

Virulent: degree or intensity of pathogenicity of an organism Virulent: degree or intensity of pathogenicity of an organism. It is indicated by mortality rate from the related disease and/or ability to invade tissue and cause disease.

Infectious Disease Process In order for a pathogen to cause disease they must be able to: Enter the host body Adhere to specific host cells Invade and colonize host tissues Inflict damage on host tissue

Entry to the host is usually through a natural orifice (mouth, eyes, genitals, wounds) Attach to specific cells to invade other body tissues Some pathogens inflict damage by growing; others produce destructive toxins

Modes of Transmission May be transmitted through direct or indirect contact Direct contact: occurs when an individual is infected by contact with the reservoir Indirect contact: occcurs when a pathogen can withstand the environment outside its host for a long period of time before infecting another individual Horizontal transmission: agent is passed from person to person Vertical transmission: agent is transmitted from mother to baby

Disease reservoir: the environment where the infectious agent survives Examples: humans or animals Clostridium tetani disease reservoir is the soil Yersenia Pestis is rodents

Microbes that cause infectious diseases Bacteria: unicellular prokaryotic organism (no organized internal membranous structure) that reproduce by growing and dividing into two cells a. bacillus (rod-shaped) b. coccus (spherical) c. spirillum (helical rods) Virus: infectious agent composed of a protein coat and a single type of nucleic acid. Lacks an independent metabolism and reproduces only within a host cell

Fungi: eukaryotic with rigid cellulose walls and reproduce primarily by forming spores Protozoa: unicellular eukaryotes capable of a variety of rapid and flexible movements Helminthes: simple, invertebrate animals that are infectious parasites. Drugs that kill helminthes are frequently toxic to humans. Prions: infectious particles that consist only of proteins. Infections usually result in brain tissue that is riddled with holes.