Bacteria are single-celled organisms that reproduce by binary fission, in which identical copies of the original cell are produced.

Viruses require living cells in order to multiply. The viruses invade cells and redirect the host organelles from their normal functions to instead replicate the virus. Fungi are very different from bactria in that their structure is more complex (eukaryotic) and they reproduce by budding (among other mechanisms). Protozoa include unicellular animal-like microorganisms. Common protozoan infections include malaria and sleeping sickness. Helminths are worm-like animals, including pinworms and tapeworms. 

Endospores are produced by gram-positive bacteria and can lie dormant for years. Endospores are crucial to growth of gram-positive bacteria because they resist harsh conditions such as extreme temperatures, ultraviolet radiation, and many chemicals. Endospores are not used by fungi by any means. They are also uninvolved in the reproductive processes of bacteria. 

The main reaction that happens during a gram stain is the detection of the presence of peptidoglycan. Most bacterial cell walls contain peptidoglycan, but in varying amounts. If the cell contains large amounts of peptidoglycan, it will retain the crystal violet dye and display a purple coloration (this is gram-positive). If the cell does not contain a large quantity of peptidoglycan, it will display the counterstain and appear pink (this is gram-negative).

The gram-stain does not dictate whether or not a species belongs to the domain archaea. Archaea cell walls do not contain peptidoglycan and will not react to the stain. Note that archaea and bacteria are different domains altogether, though they share some similar characteristics.

There are three principal forms of bacteria. Spherical bacteria are termed cocci. Rod-shaped bacteria are called bacilli. Spiral bacteria are rigid (spirilla), flexible (spirochetes), or curved (vibrios). 

Streptococcus, the causative agent of strep throat, is a genus of gram-postitive (purple staining) cocci (spheres), in which the cells tend to form chains. Bacteria are described by a combination of their response to gram stain and their appearance. For example, "gram-positive streptococcus" indicate chains of spheres that stain purple. 

The peptidoglycan layer of bacteria retains the Gram stain. In general, Gram-negative bacteria are more virulent than Gram-positive bacteria. This is due in part to an outer layer of lipopolysaccharides present on Gram-negative bacteria that invokes immune responses (particularly via a component called lipid-A), and are somewhat more difficult to target with antibiotics.

Though all of these answers are true of at least some protists, the most generally applicable statement is that protists generally prefer to live in environments that contain plenty of water. Species within the kingdom Protista are believed to have given rise to the other kingdoms of Eukarya (these other kingdoms being Fungi, Animals and Land Plants), so the incredible diversity found within Protista is not particularly surprising. 

All of these answers are true, except the statement that protists are definitionally unicellular. Protists are an incredibly diverse group of organisms that are difficult to classify because of the vast differences that exist between protist species; it is easier to classify protists based on what they are not, rather than what they are. Multicellular protists include species of seaweeds called kelp. Though kelp resemble plants, they are actually multicellular groups of protists that lack true stems, roots or leaves (though kelp often have structures resembling these plant structures).

Endosymbiosis is an intriguing theory that seeks to explain the presence of complex structures such as mitochondria and chloroplasts inside of plant and animal cells. Mitochondria's similarities to free-living proteobacteria, and chloroplasts' similarities to free-living cyanobacteria suggest that at one point these structures were organisms that became engulfed by larger cells. According to the theory of endosymbiosis, mitochondria and chloroplasts lived inside their hosts in a symbiotic relationship, eventually evolving to lose their capacity to live independently and becoming organelles that support their host cell.

The presence of chloroplasts indicate that Euglena is able to photosynthesize. These organelles contain pigments that are necessary for the conversion of light energy to chemical energy. The ability to use non-chemical energy (in this case light) to generate chemical energy is the defining characteristic of autotrophs.

The red eye spot, while it is involved in the complex of organelles that sense light, gives no information about trophic mode: many organisms sense and respond to light, but are not autotrophic.

For the most part, protists are very valuable to humans. They form the base of a large portion of aquatic food chains, and without them many ecosystems would collapse. However, some protist species have caused—and continue to cause—profound problems to humans. Of the given answers, the only one not caused by a species of protist is AIDS. AIDS is caused by a retrovirus, which means that it is an RNA virus that uses reverse transcription to replicate itself in a host cell.