Wilhelm Ostwald, Nobel Laureate in Chemistry in 1909, was one of the founders of modern physical chemistry. He is reported to have said, "There are no sharp differences between mechanical suspensions, colloidal solutions, and molecular [true] solutions. There is a gradual and continuous transition from the first through the second to the third."

A mixture, regardless of type, is described as "uniformly dispersed." This means that one or more minor components are evenly distributed throughout a major component.  The major component is the substance that is present in the greatest proportion. In the biology laboratory, the major component often is a liquid, and minor components can be solids, other liquids, or even gases.

The "mechanical suspension" to which Ostwald referred is the easiest to describe. The minor component in a suspension is typically visible in an optical microscope and often is visible to the naked eye.

A colloidal mixture is sometimes called a colloidal system, a colloidal suspension, or simply a "colloid." The smallest dimension of the minor component of a colloidal mixture can range from approximately one nanometer (1 billionth of a meter) to one micrometer (1 millionth of a meter). Examples of liquid colloidal mixtures are milk, paints, and muddy water. The medium can be a gas, in the cases of smog, smoke, or aerosol sprays.  Some solids are considered to be colloidal mixtures, as in steel or foam rubber.

In a true solution, one or minor components interact at the molecular level or ionic level with the major component. The minor components are atoms or molecules, and are not distinguishable in any optical microscope.

Most disease-causing organisms, or pathogens, are too small to be seen without a microscope. Some (e.g., most viruses) are even too small to be visible under a light microscope and must be viewed with the more powerful electron microscope. Because of their microscopic size, these minute organisms often are referred to as microbes or microorganisms. The study of these organisms is called microbiology, and scientists who study these organisms are microbiologists. Not all microbes cause disease; many are beneficial and even essential. Bacteria, in the digestive system, for example are important partners in digestion. Microbes that cause disease are sometimes informally referred to as “germs” or “bugs”.

The five main groups of pathogens are bacteria, viruses, protozoa, fungi, and helminths. Bacteria are simple, single-celled organisms that lack an organized nucleus or membrane enclosed organelles. They often have a cell wall (prokaryotes), and their cells usually are rod-shaped or spherical. Commonly known diseases caused by bacteria are diarrheal diseases, pneumonia, strep throat, tuberculosis, and anthrax. 

Viruses are particles of nucleic acid (DNA or RNA) surrounded by a protective coat that replicate within specific host cells and can spread from cell to cell. Infectious diseases caused by viruses include the flu, the common cold, AIDS, chickenpox, and hepatitis. 

Protozoa are single-celled, motile, eukaryotic organisms, found in the Kingdom Protista, that can be human parasites. A protozoan known as Plasmodium (over 170 species), causes malaria, an infectious disease that is one of the world’s top killers.

Fungi are made of eukaryotic cells (organized nucleus and membrane enclosed organelles). All fungi, with the exception of the yeast group, are multi-cellular organisms that absorb nutrients from the environment. Fungi can cause athlete’s foot, sinusitis, skin diseases, and vaginal infections.

Helminths (worms and flukes) are invertebrate animals, some of which are parasitic. Wuchereia bancrofti is transmitted to humans by way of the mosquito. The mature adults pass into lymphatic glands, obstructing lymphatic drainage and resulting in a disfiguring condition, known as elephantiasis.

Viruses are submicroscopic particles that can be seen only with a powerful electron microscope. They are not cells, but consist of genetic material, either DNA or RNA, that is enclosed in a protective layer of protein. Viruses are able to enter certain host cells and, in fact, must invade a host cell to reproduce. Once inside living cells, viruses hijack cellular apparatuses necessary to make copies of themselves. For this reason, they are considered intracellular parasites. Viruses use the instructions contained in their genomes to produce virus components, both nucleic acid and proteins, which are then assembled to produce new virus particles. The new virus particles then can go on to infect other cells and continue the cycle of virus reproduction.

In the late 1800s, filter apparatuses were devised that could remove bacteria (the smallest known organisms), from a liquid. It was not known at that time that smaller infectious agents existed that could pass through the filter. The Russian scientist, Dimitri Ivanowsky, studying the plant tobacco mosaic disease (TMV*), which affect plants, discovered that when he applied sap from infected tobacco leaves to the filter, the liquid that passed through still was infectious. However, he did not recognize that he had discovered a new type of infectious agent.

This insight was obtained six years later by the Dutch scientist, Martinus Beijerinck, who discovered that the infectious agent which passed through the filter could reproduce but would not grow on Petri dishes used to cultivate bacteria. He further realized that these agents required the presence of a host cell to reproduce. He named the agent responsible for tobacco mosaic disease a virus, after the Latin term for poison. He thought that this agent must be much smaller and simpler than bacteria. The subsequent crystallization and electron microscope images obtained by the American scientist, Wendell Stanley, in 1935 confirmed this, and the agent was named tobacco mosaic virus.

*Tobacco mosaic virus causes stunted plant growth and mottled, discolored plant leaves, especially in tobacco and other members of the tomato family (Solanceae).

With a few exceptions, viruses are too small (in the range of 10 to 200 nm) to be seen with a light microscope, so virologists use the more powerful electron transmission microscope to study them. The higher resolutions, facilitate identification of the type of virus and its structure. When there is a new outbreak of disease, such as during the 2003 SARS outbreak scientists can examine the unknown agent under an electron microscope and obtain clues about the virus with which they are dealing. Knowing the structure of a virus also is very helpful when designing drugs and vaccines to combat the agent.

Scientists prefer to be able to grow viruses in cell culture (cells derived from another organism and grown in controlled conditions), where it is much easier to study the virus. Some viruses, however, will not grow in cell culture using existing techniques (the techniques may work, but sometimes it takes awhile to find the right cell culture system). To study these viruses, virologists must infect susceptible animals to obtain enough of the agent to conduct their research.

Virologists study the genetics of viruses, how viruses cause disease, and how they interact with components within the host cell and the host’s immune system. Wherever possible, scientists also mutate viral genes to determine their functions in the virus life cycle. Using all these lines of research, virologists work to understand viruses, especially those which cause disease, and to gain information that can be applied to the design of antiviral drugs and vaccines. Particularly when working with potentially deadly viruses, virologists must adhere to strict safety procedures and perform manipulations in laboratories specially designed to contain the viruses.