Viruses can even pass through filters of unglazed porcelain. That's how they were discovered in the first place.
Skepzilla is correct. Here's the full story.
In 1884 C. Chamberland, in Pasteur's lab, discovered that if you passed a liquid containing bacteria through an unglazed PORCELAIN tube, the bacteria were COMPLETELY RETAINED and the solution that passed through (the FILTRATE) was sterile. The advantages of this tool were immediately apparent, for with it one could sterilize solutions containing heat-sensitive components by filtration through sterile porcelain tubes into sterile containers. By carefully controlling the components of the porcelain tubes you could CONTROL THE PORE SIZE and selectively remove larger organisms while letting smaller ones pass through.
This type of filtration immediately became one means of testing the Germ Theory, since if you passed an infected sample through a filter that would hold back all microbes, the filtrate should not induce the disease in a new host if a microbe was responsible. You could then begin to devise ways of growing the suspected pathogen. However, in 1892 D. IWANOWSKI applied this test to a filtrate of plants suffering from TOBACCO MOSAIC DISEASE with shocking results; the filtrate was FULLY CAPABLE of producing the ORIGINAL DISEASE in new hosts. When repeated, filtrations produced the same results and nothing could be seen in the filtrates using the most powerful microscopes, nor could anything be cultivated from the filtrates, Iwanowski and associates concluded that they had discovered a new pathogenic life-form which they called by the unimaginative, but functional, name of "FILTERABLE VIRUS". We now know that viruses range in size from 20 nm (10-9 meters) to 250 nm. We also know that some of the smallest bacteria such as the chlamydia and mycoplasma are almost as small as the largest viruses and that they too can pass through filters that retard 99% of the other bacteria.
By the early 1900 diseases like foot-mouth-disease in cattle, some cancers (in animals) and yellow fever in humans had been demonstrated to be caused by filterable viruses. The scientific community knew that it had a new group of dangerous pathogens to contend with. The term "VIRUSES" became permanently associated with this life form. You have previously seen that bacterial viruses (view the "Single Burst Graph Animation") or #bacteriophage (phage) were discovered in 1915 & 1917. Viruses, however were not "seen" until the electron microscope was developed in the late 1930s. This site contains electron micrographs of many bacteriophage; view the P2, lambda and P4 phage.
Figure 2. T-EVEN PHAGE. This is a large bacteriophage. It happens to be one of the most complex viruses. Not all phage are large; some are composed of only 7 genes. This is an E. coli phage and it has been studied intensely and much is known about it.
We now know that viruses exist that attack perhaps every form of cellular life on this planet. I haven't seen references to thermophilic phage, but I would be surprised if they didn't exist. We are discovering new viruses all the time and most virologists feel we have only scratched the surface of viral variety. For example, when sea water is concentrated and examined under the electron microscope it teems with VIRUS-LIKE PARTICLES. We have no idea what they are or where they come from or what their hosts are.
The nature of viruses became even more confusing when it was observed in 1935 that they could be CRYSTALLIZED like inorganic salts (table salt) and protein molecules. This observation started a spirited, but rather barren, argument as to whether viruses are really "alive" or a "form of life". People have argued that viruses are like salt crystals that grow and reproduce (sort of). In my view this discussion is a waste of time by people who need to "get-a-life". Viruses clearly REPLICATE their genetic material, which like that of all other life forms, is composed of nucleic acid polymers. Viruses have one major characteristic in common: they are OBLIGATE INTRACELLULAR PARASITES. Viruses are UNABLE to grow and reproduce OUTSIDE OF A LIVING CELL. Therefore their survival is absolutely dependent upon the continued survival of their hosts. This poses an interesting dilemma for pathogens that often as not kill their hosts, wouldn't you say?
The intracellular nature of viruses presents a challenge for the investigator who must not only grow the virus but also be able to cultivate the virus' host cell. With plant and bacterial viruses it was possible to extract sufficient virus from an infected host to do analysis on it. These studies showed that viruses were mainly COMPOSED OF PROTEIN AND NUCLEIC ACID. With multicellular eukaryotic viruses the field of virus investigation could only move as rapidly as the advancements in eukaryotic TISSUE CULTURING. The first breakthrough in this problem came with the discovery in 1931 that the fertilized hen's eggs could serve as a "petri dish" for some viruses. This capacity led to the first use of artificially cultivated viruses for vaccine production. Even today many viruses are grown on eggs because they are relatively inexpensive and because the techniques are so well established.
With the discovery of the electron microscope it became possible to study the morphology of viruses. It was quickly realized that the size and shape of an individual virus is a constant and distinguishing characteristic. Therefore a virus's size and shape is always part of its description. Viruses may consist of circles, ovals, long thick or thin rods, flexible or stiff rods and ones with distinctive heads and tail components. The smallest viruses are around 20 nm in diameter and the largest around 250 nm.
Some electron micrographs and computer renderings of viruses are seen at the following Internet addresses :
These were all functional as of 2/26/99:
Lots of colorized viruses (Psst!! the viruses are really these colors folks)
Many good EM pictures of virus types
Picture of Ebola virus
Lots of very good virus pictures.
huge site for viruses with courses and virus pictures etc.
Excellent discussion of virus structure.
Pictures of plant viruses.
; Many pictures, cartoons and good explanations, but many are a bit too advanced for Micro 101/102.
More at: https://www.slic2.wsu.edu:82/hurlbert...es/Chap11.html